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/suggested_commands.md

@@ -19,14 +19,14 @@ All run from repo root (`nix-overlay/`).
 - `nix-update skillsmcp --version=branch=main` — for commit-pinned pkgs; or `--commit <sha>`.
 
 ## Beads (mandatory task tracking)
-- `bd prime` — session context, run before any work.
-- `bd ready` / `bd show <id>` / `bd update <id> --claim` / `bd close <id>`.
-- `bd remember` — persistent knowledge instead of MEMORY.md.
-- `bd dolt push` — prints "No remote is configured — skipping." (expected; exit 0).
+- `br robot-docs guide` — session context, run before any work.
+- `br ready` / `br show <id>` / `br update <id> --claim` / `br close <id>`.
+- `br update <id> --append-notes "..."` — persistent knowledge instead of MEMORY.md.
+- `br sync --flush-only` — export issue state to `.beads/issues.jsonl`; runs no git commands.
 
 ## Session completion push
 ```
-git pull --rebase && bd dolt push && git push
+git pull --rebase && br sync --flush-only && git add .beads/ && git commit -m "sync beads" && git push
 git status  # must show "up to date with origin"
 ```