chore(beads): migrate issue tracking from bd to br

Agents were told to run bd commands that beads_rust does not have (bd prime,
bd remember, bd dolt push, bd tag, bd note, bd children), and bd's git hooks
were still active, so bd could re-export its stale dolt state over br's
JSONL on commit.

- Rewrite AGENTS.md, docs/agents/issue-tracker.md, docs/agents/triage-labels.md
  and the .serena memories around br's real CLI surface
- Replace bd's generated AGENTS.md block with br's (br agents --add)
- Point the .claude session hooks at `br robot-docs guide`
- Remove bd git hooks and unset core.hooksPath; br installs no hooks
- Ignore br runtime files (local SQLite DB, recovery and merge artifacts)
- Rewrite .beads/README.md, config.yaml and metadata.json for br

Issue: nix-overlay-6pp

.beads/.gitignore

@@ -1,7 +1,22 @@
-# Dolt database (managed by Dolt, not git)
-dolt/
+# Database (local only, not git)
+# br migrated the bd dolt database to a local SQLite file that kept the name "dolt"
+dolt*
 embeddeddolt/
 
+# br (beads_rust) runtime artifacts
+.br_recovery/
+.br_history/
+.br-wal-index-*/
+
+# br merge artifacts (temporary files from 3-way merge)
+beads.base.jsonl
+beads.base.meta.json
+beads.left.jsonl
+beads.left.meta.json
+beads.right.jsonl
+beads.right.meta.json
+sync_base.jsonl
+
 # Runtime files
 bd.sock
 bd.sock.startlock