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/core.md

@@ -23,8 +23,8 @@ Repo: `git+https://git.millerson.name/alex/millerson-overlay.nix.git`
 Runnable commands (build/format/update/bd/push): `mem:suggested_commands`. Exact done-checklist: `mem:task_completion`.
 
 
-- ALL task tracking via `bd` (beads): `bd ready`, `bd update <id> --claim`, `bd close <id>`. NEVER TodoWrite/markdown TODOs.
-- `bd prime` before starting work; `bd remember` for persistent knowledge (no MEMORY.md files).
-- Session not complete until `git push` succeeds (`bd dolt push` skipping with "No remote is configured" is expected, not a failure).
+- ALL task tracking via `br` (beads): `br ready`, `br update <id> --claim`, `br close <id>`. NEVER TodoWrite/markdown TODOs.
+- `br robot-docs guide` before starting work; persistent knowledge goes into issue notes (`br update <id> --append-notes "..."`) instead of MEMORY.md files.
+- Session not complete until `git push` succeeds. `br` never runs git: run `br sync --flush-only`, then `git add .beads/ && git commit` before the push.
 - Commits use conventional-commit format.
 - For nixpkgs/option lookups use the `nix` skill; for flake/devShell ops the `nix-flakes` skill; comms in caveman mode.