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/export.md

@@ -0,0 +1,24 @@
+---
+description: Export issues to JSONL format
+argument-hint: [-o output-file]
+---
+
+Export all issues to JSON Lines format (one JSON object per line).
+
+## Usage
+
+- **To stdout**: `bd export`
+- **To file**: `bd export -o issues.jsonl`
+- **Filter by status**: `bd export --status open`
+
+Issues are sorted by ID for consistent diffs, making git diffs readable.
+
+## Automatic Export
+
+The daemon automatically exports to `.beads/issues.jsonl` after any CRUD operation (5-second debounce). Manual export is rarely needed unless you need a custom output location or filtered export.
+
+Export is used for:
+- Git version control
+- Backup
+- Sharing issues between repositories
+- Data migration