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

.claude/settings.json

@@ -4,7 +4,7 @@
       {
         "hooks": [
           {
-            "command": "bd prime",
+            "command": "br robot-docs guide",
             "type": "command"
           }
         ],
@@ -15,7 +15,7 @@
       {
         "hooks": [
           {
-            "command": "bd prime",
+            "command": "br robot-docs guide",
             "type": "command"
           }
         ],