v0.5.1: audit fixes, skeleton command, agent safety guide

[iicky]

• Fix scoped-only secrets silently dropped on save (P0 data loss bug)
• Fix export --json emitting shell-escaped values instead of raw strings
• Fix import silently overwriting existing secrets (now requires --force)
• Fix merge driver writing stale meta when MURK_KEY unavailable
• Fix diff ignoring scoped/mote values entirely
• Fix info hiding tags without key and omitting recipient names with key
• Fix key resolution not vault-aware (info, circle bypassed key file lookup)
• Fix revoke overstating exposed keys
• Reorder integrity MAC check before decryption for better error messages
• Update deny.toml RSA advisory comment to reflect actual ssh-rsa usage
• Add murk skeleton command: export schema-only vault with no secrets
• Add docs.rs badge to README
• Add AI agent safety guide (docs/ai-agents.md)

[codecov]

Codecov Report

❌ Patch coverage is 41.73228% with 74 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.54%. Comparing base (999f1f0) to head (510068a).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/main.rs	0.00%	64 Missing ⚠️
src/info.rs	78.94%	4 Missing ⚠️
src/lib.rs	86.20%	4 Missing ⚠️
src/export.rs	75.00%	2 Missing ⚠️

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