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>

QUICKSTART.md

@@ -129,7 +129,13 @@ You can use project-specific databases:
 After upgrading bd, use `bd migrate` to check for and migrate old database files:
 
 ```bash
-# Check for migration opportunities
+# Inspect migration plan (AI agents)
+./bd migrate --inspect --json
+
+# Check schema and config
+./bd info --schema --json
+
+# Preview migration changes
 ./bd migrate --dry-run
 
 # Migrate old databases to beads.db
@@ -139,6 +145,8 @@ After upgrading bd, use `bd migrate` to check for and migrate old database files
 ./bd migrate --cleanup --yes
 ```
 
+**AI agents:** Use `--inspect` to analyze migration safety before running. The system verifies required config keys and data integrity invariants.
+
 ## Next Steps
 
 - Add labels: `./bd create "Task" -l "backend,urgent"`