refactor: drop steer/rewind extensions removed in zcode 0.16+, release 0.10.0

[william0wang]

Summary

The zcode app-server removed three RPC methods in 0.16+ (verified live against 0.16.3, bundled in ZCode desktop 3.8.1): session/steer, session/rewind, session/rewindCascade all answer -32601 Method not found — steer/rewind moved to the v4 conversation API, which this bridge does not speak.

Audit showed near-zero external usage (non-standard ACP extensions no editor calls; rewindCascade had no slash command either), and on 0.16+ the passthroughs only ever returned errors. So this drops them rather than gating them.

• Remove the three ACP extension registrations and handlers (session/fork stays as the checkpoint-branching alternative)
• Remove the /rewind and /steer slash commands and their advertisement
• Refresh docs/BACKLOG.md to a 0.16.3 audit: upstream removals, new candidates (workspace/hooks/trustGrant, interaction/browser*), new session/send params (attachments, toolDenylist, runtimeModel, …), v4 protocol family
• PROTOCOL.md / README version tables gain a 0.16.0 row
• Release 0.10.0 (breaking removal, 0.x minor)

Verification

• pnpm typecheck, pnpm lint, pnpm test (652 tests) all pass
• Rebuilt and drove the bridge over ACP stdio: session/steer / session/rewind / session/rewindCascade now return a clean ACP-level -32601 (previously -32603 after a wasted backend round-trip); /rewind /steer no longer advertised; core flow (initialize → session/new → session/prompt) unaffected