docs: README.md — promote ERROR_HANDLING.md to first-class navigation

Cycle #23 ships a documentation discoverability fix.

After #22 shipping ERROR_HANDLING.md, the next natural step is making it
discoverable from the project's entry point (README.md).

Before: README top navigation linked to USAGE, PARITY, ROADMAP, Rust workspace.
ERROR_HANDLING.md was buried in CLAUDE.md references.

After: ERROR_HANDLING.md is now in the top navigation (right after USAGE,
before Rust workspace). Also added SCHEMAS.md mention in repository shape.

This signals that:
1. Error handling is a first-class concern (not an afterthought)
2. The Python harness documentation (SCHEMAS.md, ERROR_HANDLING.md, CLAUDE.md)
   is part of the official docs, not just dogfood artifacts
3. New users/claws can discover the error-handling pattern at entry point

Impact: Operators building orchestration code will immediately see
'Error Handling' link in navigation, shortening the path to understanding
how to consume the protocol reliably.

No code changes. No test changes. Pure navigation/discoverability.

README.md

@@ -5,6 +5,8 @@
   ·
   <a href="./USAGE.md">Usage</a>
   ·
+  <a href="./ERROR_HANDLING.md">Error Handling</a>
+  ·
   <a href="./rust/README.md">Rust workspace</a>
   ·
   <a href="./PARITY.md">Parity</a>
@@ -40,9 +42,11 @@ The canonical implementation lives in [`rust/`](./rust), and the current source
 
 - **`rust/`** — canonical Rust workspace and the `claw` CLI binary
 - **`USAGE.md`** — task-oriented usage guide for the current product surface
+- **`ERROR_HANDLING.md`** — unified error-handling pattern for orchestration code
 - **`PARITY.md`** — Rust-port parity status and migration notes
 - **`ROADMAP.md`** — active roadmap and cleanup backlog
 - **`PHILOSOPHY.md`** — project intent and system-design framing
+- **`SCHEMAS.md`** — JSON protocol contract (Python harness reference)
 - **`src/` + `tests/`** — companion Python/reference workspace and audit helpers; not the primary runtime surface
 
 ## Quick start