chore: release 0.3.0 — docs sync and version bump

[william0wang]

Summary

Post-merge documentation sync for the remote-access feature set, plus the
0.3.0 version bump.

What's in here

• CHANGELOG: [Unreleased] → [0.3.0] - 2026-08-17 (remote access, hub
  daemon, tail replay, account/usage_stats, slash/reminder fixes)
• README (en): refreshed Status; architecture layer list + remote/ +
  quota/; Discovery API block gains ?probe=1 and account/usage_stats;
  docs index links Remote Clients + Replay Guide
• README.zh-CN: adds the entire 远程访问 section (was missing) and the
  five remote env-var rows
• DEVELOPMENT: Method 4 — local remote-access testing commands
• TROUBLESHOOTING: hub unreachable/401 and stale-instance entries
• AGENTS.md: handlers/replay.ts tree row; REPLAY-GUIDE: note that
  replayed user messages strip <system-reminder> blocks
• Version: package.json 0.2.0 → 0.3.0; registry agent.json 0.1.0 → 0.3.0

Verification

568 tests passing, typecheck/lint/build clean; AGENT_INFO.version reads
0.3.0 from package.json after build.