docs: add v3.0 release notes and update meta files for hardening sprint
- docs/v3-release-notes.md: comprehensive v3.0 release notes covering
Mission Control, multi-agent orchestration, production readiness,
multi-machine early access, test coverage, and known limitations
- docs/v3-progress.md: hardening sprint session entry
- CHANGELOG.md: security entries (TLS, WAL, plugin sandbox, Landlock)
and bug fixes (subagent delegation, gitignore)
- TODO.md: hardening complete, remaining items moved to v3.1
- CLAUDE.md: updated test counts (444 vitest + 111 cargo)
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