Fix bd init to auto-import issues from git on fresh clone

Prevents agents from creating duplicate low-numbered issues when starting
with a fresh git clone that already has issues.jsonl in git history.

Changes:
- bd init now checks for existing issues in git after DB creation
- Auto-imports with collision resolution if found
- Updates AGENTS.md to simplify onboarding (just 'bd init')

Fixes the scenario where:
1. Fresh clone has .beads/issues.jsonl in git (212 issues)
2. Agent runs bd init (creates empty DB)
3. Agent starts creating bd-1, bd-2, etc (collisions with git)

Now bd init automatically imports all issues from git on first run.

Amp-Thread-ID: https://ampcode.com/threads/T-8a41f14d-d4c3-4c50-a18b-5f112110f138
Co-authored-by: Amp <amp@ampcode.com>

AGENTS.md

@@ -456,11 +456,9 @@ Happy coding! 🔗
 
 ### Quick Start
 
-**FIRST TIME?** Import existing issues from git:
+**FIRST TIME?** Just run `bd init` - it auto-imports issues from git:
 ```bash
-bd import -i .beads/issues.jsonl --json
-# Or if issues.jsonl is empty in working tree but exists in git:
-git show HEAD:.beads/issues.jsonl | bd import -i /dev/stdin --json
+bd init --prefix bd
 ```
 
 **Check for ready work:**