refactor: move Claude Code plugin to dedicated subdirectory (GH#985)

Move all plugin content to claude-plugin/ subdirectory for cleaner
separation from core beads functionality:

- claude-plugin/.claude-plugin/plugin.json - plugin manifest
- claude-plugin/commands/ - all slash commands
- claude-plugin/agents/ - task-agent (now in correct location)
- claude-plugin/skills/beads/ - beads skill with resources

Root .claude-plugin/ now only contains marketplace.json pointing
to ./claude-plugin.

Note: After updating, users may need to clear plugin cache:
  rm -rf ~/.claude/plugins/cache/beads-marketplace/

Fixes #985

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Executed-By: beads/crew/dave
Rig: beads
Role: crew

claude-plugin/commands/audit.md

@@ -0,0 +1,28 @@
+---
+description: Log and label agent interactions (append-only JSONL)
+argument-hint: record|label
+---
+
+Append-only audit logging for agent interactions (prompts, responses, tool calls) in `.beads/interactions.jsonl`.
+
+Each line is one event. Labeling is done by appending a new `"label"` event referencing a previous entry.
+
+## Usage
+
+- **Record an interaction**:
+  - `bd audit record --kind llm_call --model "claude-3-5-haiku" --prompt "..." --response "..."`
+  - `bd audit record --kind tool_call --tool-name "go test" --exit-code 1 --error "..." --issue-id bd-42`
+
+- **Pipe JSON via stdin**:
+  - `cat event.json | bd audit record`
+
+- **Label an entry**:
+  - `bd audit label int-a1b2 --label good --reason "Worked perfectly"`
+  - `bd audit label int-a1b2 --label bad --reason "Hallucinated a file path"`
+
+## Notes
+
+- Audit entries are **append-only** (no in-place edits).
+- `bd sync` includes `.beads/interactions.jsonl` in the commit allowlist (like `issues.jsonl`).
+
+