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

.serena/memories/task_completion.md

@@ -7,5 +7,5 @@ Done when ALL of these pass/hold:
 3. New package is staged (`git add packages/<name>/`) before build — blueprint index requirement.
 4. README.md Available Packages table updated if package added/changed visibly.
 5. Commit: conventional commit format (use conventional-commit / caveman-commit skill). No work left uncommitted.
-6. Beads: work claimed via `bd update <id> --claim` before, `bd close <id>` after.
-7. Pushed: `git pull --rebase && bd dolt push && git push` then `git status` shows "up to date with origin". `bd dolt push` skip message is expected, not a failure. Do not stop before push succeeds.
+6. Beads: work claimed via `br update <id> --claim` before, `br close <id>` after.
+7. Pushed: `git pull --rebase && br sync --flush-only && git add .beads/ && git commit -m "sync beads" && git push` then `git status` shows "up to date with origin". Do not stop before push succeeds.