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/import.md

@@ -0,0 +1,36 @@
+---
+description: Import issues from JSONL format
+argument-hint: [-i input-file]
+---
+
+Import issues from JSON Lines format (one JSON object per line).
+
+## Usage
+
+- **From stdin**: `bd import` (reads from stdin)
+- **From file**: `bd import -i issues.jsonl`
+- **Preview**: `bd import -i issues.jsonl --dry-run`
+
+## Behavior
+
+- **Existing issues** (same ID): Updated with new data
+- **New issues**: Created
+- **Same-ID scenarios**: With hash-based IDs (v0.20.1+), same ID = same issue being updated (not a collision)
+
+## Preview Changes
+
+Use `--dry-run` to see what will change before importing:
+
+```bash
+bd import -i issues.jsonl --dry-run
+# Shows: new issues, updates, exact matches
+```
+
+## Automatic Import
+
+The daemon automatically imports from `.beads/issues.jsonl` when it's newer than the database (e.g., after `git pull`). Manual import is rarely needed.
+
+## Options
+
+- **--skip-existing**: Skip updates to existing issues
+- **--strict**: Fail on dependency errors instead of warnings