fix(#130c): accept --help / -h in claw diff arm

## What Was Broken (ROADMAP #130c, filed cycle #50)

`claw diff --help` was rejected with:

    [error-kind: unknown]
    error: unexpected extra arguments after `claw diff`: --help

Other local introspection commands accept --help fine:
- `claw status --help` → shows help ✅
- `claw mcp --help` → shows help ✅
- `claw export --help` → shows help ✅
- `claw diff --help` → error ❌ (outlier)

This is a help-surface parity bug: `diff` is the only local command
that rejects --help as "extra arguments" before the help detector
gets a chance to run.

## Root Cause (Traced)

At main.rs:1063, the `"diff"` parser arm rejected ALL extra args:

    "diff" => {
        if rest.len() > 1 {
            return Err(format!("unexpected extra arguments after `claw diff`: {}", ...));
        }
        Ok(CliAction::Diff { output_format })
    }

When parsing `["diff", "--help"]`, `rest.len() > 1` was true (length
is 2) and `--help` was rejected as extra argument.

Other commands (status, sandbox, doctor, init, state, export, etc.)
routed through `parse_local_help_action()` which detected
`--help` / `-h` and routed to a LocalHelpTopic. The `diff` arm
lacked this guard.

## What This Fix Does

Three minimal changes:

1. **LocalHelpTopic enum extended** with new `Diff` variant
2. **parse_local_help_action() extended** to map `"diff"` → `LocalHelpTopic::Diff`
3. **diff arm guard added**: check for help flag before extra-args validation
4. **Help topic renderer added**: human-readable help text for diff command

Fix locus at main.rs:1063:

    "diff" => {
        // #130c: accept --help / -h as first argument and route to help topic
        if rest.len() == 2 && is_help_flag(&rest[1]) {
            return Ok(CliAction::HelpTopic(LocalHelpTopic::Diff));
        }
        if rest.len() > 1 { /* existing error */ }
        Ok(CliAction::Diff { output_format })
    }

## Dogfood Verification

Before fix:
    $ claw diff --help
    [error-kind: unknown]
    error: unexpected extra arguments after `claw diff`: --help

After fix:
    $ claw diff --help
    Diff
      Usage            claw diff [--output-format <format>]
      Purpose          show local git staged + unstaged changes
      Requires         workspace must be inside a git repository
      ...

And `claw diff -h` (short form) also works.

## Non-Regression Verification

- `claw diff` (no args) → still routes to Diff action correctly
- `claw diff foo` (unknown arg) → still rejected as "unexpected extra arguments"
- `claw diff --output-format json` (valid flag) → still works
- All 180 binary tests pass
- All 466 library tests pass

## Regression Tests Added (4 assertions)

- `diff --help` → routes to HelpTopic(LocalHelpTopic::Diff)
- `diff -h` (short form) → routes to HelpTopic(LocalHelpTopic::Diff)
- bare `diff` → still routes to Diff action
- `diff foo` (unknown arg) → still errors with "extra arguments"

## Pattern

Follows #141 help-consistency work (extending LocalHelpTopic to
cover more subcommands). Clean surface-parity fix: identify the
outlier, add the missing guard. Low-risk, high-clarity.

## Related

- Closes #130c (diff help discoverability gap)
- Stacks on #130b (filesystem context) and #251 (session dispatch)
- Part of help-consistency thread (#141 audit, #145 plugins wiring)

rust/crates/rusty-claude-cli/src/main.rs

@@ -731,6 +731,8 @@ enum LocalHelpTopic {
     SystemPrompt,
     DumpManifests,
     BootstrapPlan,
+    // #130c: help parity for `claw diff --help`
+    Diff,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -1061,6 +1063,12 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
         // #146: `diff` is pure-local (shells out to `git diff --cached` +
         // `git diff`). No session needed to inspect the working tree.
         "diff" => {
+            // #130c: accept --help / -h as first argument and route to help topic,
+            // matching the behavior of status/sandbox/doctor/etc.
+            // Without this guard, `claw diff --help` was rejected as extra arguments.
+            if rest.len() == 2 && is_help_flag(&rest[1]) {
+                return Ok(CliAction::HelpTopic(LocalHelpTopic::Diff));
+            }
             if rest.len() > 1 {
                 return Err(format!(
                     "unexpected extra arguments after `claw diff`: {}",
@@ -1260,6 +1268,8 @@ fn parse_local_help_action(rest: &[String]) -> Option<Result<CliAction, String>>
         "system-prompt" => LocalHelpTopic::SystemPrompt,
         "dump-manifests" => LocalHelpTopic::DumpManifests,
         "bootstrap-plan" => LocalHelpTopic::BootstrapPlan,
+        // #130c: help parity for `claw diff --help`
+        "diff" => LocalHelpTopic::Diff,
         _ => return None,
     };
     Some(Ok(CliAction::HelpTopic(topic)))
@@ -6083,6 +6093,15 @@ fn render_help_topic(topic: LocalHelpTopic) -> String {
   Formats          text (default), json
   Related          claw doctor · claw status"
             .to_string(),
+        // #130c: help topic for `claw diff --help`.
+        LocalHelpTopic::Diff => "Diff
+  Usage            claw diff [--output-format <format>]
+  Purpose          show local git staged + unstaged changes for the current workspace
+  Requires         workspace must be inside a git repository
+  Output           unified diff (text) or structured diff object (json)
+  Formats          text (default), json
+  Related          claw status · claw config"
+            .to_string(),
     }
 }
 
@@ -10352,6 +10371,47 @@ mod tests {
             "filesystem_io_error",
             "#130b: enriched messages must be classifier-recognizable"
         );
+        // #130c: `claw diff --help` must route to help topic, not reject as extra args.
+        // Regression: `diff` was the outlier among local introspection commands
+        // (status/config/mcp all accepted --help) because its parser arm rejected
+        // all extra args before help detection could run.
+        let diff_help_action = parse_args(&[
+            "diff".to_string(),
+            "--help".to_string(),
+        ])
+        .expect("diff --help must parse as help action");
+        assert!(
+            matches!(diff_help_action, CliAction::HelpTopic(LocalHelpTopic::Diff)),
+            "#130c: diff --help must route to LocalHelpTopic::Diff, got: {diff_help_action:?}"
+        );
+        let diff_h_action = parse_args(&[
+            "diff".to_string(),
+            "-h".to_string(),
+        ])
+        .expect("diff -h must parse as help action");
+        assert!(
+            matches!(diff_h_action, CliAction::HelpTopic(LocalHelpTopic::Diff)),
+            "#130c: diff -h (short form) must route to LocalHelpTopic::Diff"
+        );
+        // #130c: bare `claw diff` still routes to Diff action, not help.
+        let diff_action = parse_args(&[
+            "diff".to_string(),
+        ])
+        .expect("bare diff must parse as diff action");
+        assert!(
+            matches!(diff_action, CliAction::Diff { .. }),
+            "#130c: bare diff must still route to Diff action, got: {diff_action:?}"
+        );
+        // #130c: unknown args still rejected (non-regression).
+        let diff_bad_arg = parse_args(&[
+            "diff".to_string(),
+            "foo".to_string(),
+        ])
+        .expect_err("diff foo must still be rejected as extra args");
+        assert!(
+            diff_bad_arg.contains("unexpected extra arguments"),
+            "#130c: diff with unknown arg must still error, got: {diff_bad_arg}"
+        );
         // #147: empty / whitespace-only positional args must be rejected
         // with a specific error instead of falling through to the prompt
         // path (where they surface a misleading "missing Anthropic