Document migration inspection commands in user docs

- Add bd migrate --inspect and bd info --schema examples to README.md
- Update QUICKSTART.md migration section with AI agent workflow
- Expand AGENTS.md with migration safety invariants explanation
- Clarify when and why to use inspection before migrating

Makes the new AI-supervised migration features discoverable.

Amp-Thread-ID: https://ampcode.com/threads/T-de7e1141-87ac-4b4a-9cea-1b7bc4d51da9
Co-authored-by: Amp <amp@ampcode.com>

README.md

@@ -259,6 +259,12 @@ Hash IDs use **birthday paradox probability** to determine length:
 **Existing databases continue to work** - no forced migration. Run `bd migrate` when ready:
 
 ```bash
+# Inspect migration plan (for AI agents)
+bd migrate --inspect --json
+
+# Check schema and config state
+bd info --schema --json
+
 # Preview migration
 bd migrate --dry-run
 
@@ -269,6 +275,8 @@ bd migrate
 bd info
 ```
 
+**AI-supervised migrations:** The `--inspect` flag provides migration plan analysis for AI agents. The system verifies data integrity invariants (required config keys, foreign key constraints, issue counts) before committing migrations.
+
 **Note:** Hash IDs require schema version 9+. The `bd migrate` command detects old schemas and upgrades automatically.
 
 ### Hierarchical Child IDs