docs: improve README scannability and document the exit-code contract

[ThomasK33]

Improves README scannability and documents the CLI's stable exit-code contract, inspired by a side-by-side review of coder/boo's README.

What changed

README.md

• Added an anchor nav line under the badges (Quickstart · Why · How it works · Commands · Contributing).
• Framed the Quickstart example as "the canonical loop" with numbered step comments (open → type → wait → read → capture → clean up).
• Rewrote the "Command surface" section from a dense prose paragraph into a grouped table (lifecycle / input / observation / live view / environment), added a stable-exit-code note, and pointed readers at --help self-discoverability and the new exit-code reference.

docs/USAGE.md

• New Exit Codes section documenting the full 0–11 mapping from src/cli/exitCodes.ts, plus batch fail-fast / --keep-going semantics.
• New note in the wait section: a standalone wait that times out exits 0 with timedOut: true in the JSON result; exit code 11 (WAIT_TIMEOUT) only surfaces from a timed-out wait step inside a fail-fast batch. Callers must check the envelope, not the exit code.

src/cli/exitCodes.ts

• One-line comment marking the exit-code map as a public contract documented in docs/USAGE.md, so code and docs don't drift.

No behavior changes — docs plus one source comment.

Validation

• mise run ci (format check, lint, typecheck, tests).
• Exit codes verified empirically against a live session in an isolated AGENT_TTY_HOME: usage error → 2, session not found → 3, destroy → 0, already destroyed → 4, and standalone wait timeout → 0 with {"matched": false, "timedOut": true} in the envelope (which is why the docs describe 11 as batch-only — confirmed in src/batch/executor.ts and test/integration/batch.test.ts).

Note for maintainers

The standalone-wait-exits-0-on-timeout behavior is now documented as-is, not changed. If you'd rather standalone wait also exit non-zero on timeout (so wait ... && next-step fails closed), that's a breaking CLI change worth its own issue.

---

Generated with mux • Model: anthropic:claude-fable-5 • Thinking: xhigh