docs: ERROR_HANDLING.md — unified error handler pattern for orchestration code

Cycle #22 ships documentation that operationalizes cycles #178–#179.

Problem context:
After #178 (parse-error envelope) and #179 (stderr hygiene + real error message),
claws can now build a unified error handler for all 14 clawable commands.
But there was no guide on how to actually do that. Operators had the pieces;
they didn't have the pattern.

This file changes that.

New file: ERROR_HANDLING.md
- Quick reference: exit codes + envelope shapes (0=success, 1=error, 2=timeout)
- One-handler pattern: ~80 lines of Python showing how to parse error.kind,
  check retryable, and decide recovery strategy
- Four practical recovery patterns:
  - Retry on transient errors (filesystem, timeout)
  - Reuse session after timeout (if cancel_observed=true)
  - Validate command syntax before dispatch (dry-run --help)
  - Log errors for observability
- Error kinds enumeration (parse, session_not_found, filesystem, runtime, timeout)
- Common mistakes to avoid (6 patterns with BAD vs GOOD examples)
- Testing your error handler (unit test examples)

Operational impact:
Orchestration code now has a canonical pattern. Claws can:
- Copy-paste the run_claw_command() function (works for all commands)
- Classify errors uniformly (no special cases per command)
- Decide recovery deterministically (error.kind + retryable + cancel_observed)
- Log/monitor/escalate with confidence

Related cycles:
- #178: Parse-error envelope (commands now emit structured JSON on invalid argv)
- #179: Stderr hygiene + real message (JSON mode silences argparse, carries actual error)
- #164 Stage B: cancel_observed field (callers know if session is safe for reuse)

Updated CLAUDE.md:
- Added ERROR_HANDLING.md to 'Related docs' section
- Now documents the one-handler pattern as a guideline

No code changes. No test changes. Pure documentation.

This completes the documentation trail from protocol (SCHEMAS.md) →
governance (OPT_OUT_AUDIT.md, OPT_OUT_DEMAND_LOG.md) → practice (ERROR_HANDLING.md).

CLAUDE.md

@@ -186,7 +186,10 @@ The Python harness is continuously dogfood-tested:
 
 ## Related docs
 
+- **`ERROR_HANDLING.md`** — Unified error-handling pattern for claws (one handler for all 14 clawable commands)
 - **`SCHEMAS.md`** — JSON protocol specification (read before implementing)
+- **`OPT_OUT_AUDIT.md`** — Governance for the 12 non-clawable surfaces
+- **`OPT_OUT_DEMAND_LOG.md`** — Active survey recording real demand signals (evidence base for decisions)
 - **`ROADMAP.md`** — macro roadmap and macro pain points
 - **`PHILOSOPHY.md`** — system design intent
 - **`PARITY.md`** — status of Python ↔ Rust protocol equivalence