fix: #124 --model validation rejects malformed syntax at parse time

Adds validate_model_syntax() that rejects:
- Empty strings
- Strings with spaces (e.g., 'bad model')
- Invalid provider/model format

Accepts:
- Known aliases (opus, sonnet, haiku)
- Valid provider/model format (provider/model)

Wired into parse_args for both --model <value> and --model=<value> forms.
Errors exit with clear message before any API calls (no token burn).

Verified:
- 'claw --model "bad model" version' → error, exit 1
- 'claw --model "" version' → error, exit 1
- 'claw --model opus version' → works
- 'claw --model anthropic/claude-opus-4-6 version' → works

Refs: ROADMAP #124 (debbcbe cluster — parser-level trust gap family)

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

@@ -447,11 +447,14 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
                 let value = args
                     .get(index + 1)
                     .ok_or_else(|| "missing value for --model".to_string())?;
+                validate_model_syntax(value)?;
                 model = resolve_model_alias_with_config(value);
                 index += 2;
             }
             flag if flag.starts_with("--model=") => {
-                model = resolve_model_alias_with_config(&flag[8..]);
+                let value = &flag[8..];
+                validate_model_syntax(value)?;
+                model = resolve_model_alias_with_config(value);
                 index += 1;
             }
             "--output-format" => {
@@ -1035,6 +1038,37 @@ fn resolve_model_alias_with_config(model: &str) -> String {
     resolve_model_alias(trimmed).to_string()
 }
 
+/// Validate model syntax at parse time.
+/// Accepts: known aliases (opus, sonnet, haiku) or provider/model pattern.
+/// Rejects: empty, whitespace-only, strings with spaces, or invalid chars.
+fn validate_model_syntax(model: &str) -> Result<(), String> {
+    let trimmed = model.trim();
+    if trimmed.is_empty() {
+        return Err("model string cannot be empty".to_string());
+    }
+    // Known aliases are always valid
+    match trimmed {
+        "opus" | "sonnet" | "haiku" => return Ok(()),
+        _ => {}
+    }
+    // Check for spaces (malformed)
+    if trimmed.contains(' ') {
+        return Err(format!(
+            "invalid model syntax: '{}' contains spaces. Use provider/model format or known alias",
+            trimmed
+        ));
+    }
+    // Check provider/model format: provider_id/model_id
+    let parts: Vec<&str> = trimmed.split('/').collect();
+    if parts.len() != 2 || parts[0].is_empty() || parts[1].is_empty() {
+        return Err(format!(
+            "invalid model syntax: '{}'. Expected provider/model (e.g., anthropic/claude-opus-4-6) or known alias (opus, sonnet, haiku)",
+            trimmed
+        ));
+    }
+    Ok(())
+}
+
 fn config_alias_for_current_dir(alias: &str) -> Option<String> {
     if alias.is_empty() {
         return None;