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/metadata.json

@@ -1,7 +1,4 @@
 {
   "database": "dolt",
-  "backend": "dolt",
-  "dolt_mode": "embedded",
-  "dolt_database": "nix_overlay",
-  "project_id": "bf9206b6-9430-479c-bc02-786cff5dbffc"
-}
+  "jsonl_export": "issues.jsonl"
+}