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/comments.md

@@ -0,0 +1,30 @@
+---
+description: View or manage comments on an issue
+argument-hint: [issue-id]
+---
+
+View or add comments to a beads issue.
+
+Comments are separate from issue properties (title, description, etc.) because they serve a different purpose: they're a **discussion thread** rather than **singular editable fields**. Use `bd comments` for threaded conversations and `bd edit` for core issue metadata.
+
+## View Comments
+
+To view all comments on an issue:
+- $1: Issue ID (e.g., bd-123)
+
+Use the beads CLI `bd comments <issue-id>` to list all comments. Show them to the user with timestamps and authors.
+
+## Add Comment
+
+To add a comment:
+- $1: "add"
+- $2: Issue ID
+- $3: Comment text (or use -f flag for file input)
+
+Use `bd comments add <issue-id> "comment text"` to add a comment. Confirm the comment was added successfully.
+
+Comments are useful for:
+- Progress updates during work
+- Design notes or technical decisions
+- Links to related resources
+- Questions or blockers