feat(plugin): ship as Claude Code plugin with 150-line cap and sync command

Turns ClaudeForge into an installable Claude Code plugin and adds the
missing pieces for a clean lifecycle: hard line-cap enforcement, modular
chaining via @path imports, a sync/cleanup command, and Explore-agent
delegation for project discovery.

- .claude-plugin/plugin.json: plugin manifest registering both skills,
  both commands, and the guardian agent (installable via /plugin marketplace
  add alirezarezvani/ClaudeForge && /plugin install claudeforge)
- skill/validator.py: MAX_RECOMMENDED_LINES 300 -> 150 (warning at 120)
- skill/template_selector.py: target_lines capped at 150 across all
  team sizes (solo 75 / small 100 / medium 125 / large 150) so any
  single CLAUDE.md fits within the cap; bigger projects spread content
  across modular sub-files
- skill/analyzer.py: length thresholds, quality scoring, and recommendations
  rebased on the 150 cap (was 300/400)
- skill/generator.py: modular root now emits @path imports next to the
  human-readable links; every sub-CLAUDE.md gets a back-link header
  pointing to ../CLAUDE.md (markdown + @import) for bidirectional chaining
- command/sync-claude-md.md: new /sync-claude-md command that inventories
  every CLAUDE.md, prunes stale references, enforces the 150 cap by
  splitting into sub-files, and repairs the root <-> sub chain
- command/enhance-claude-md.md: discovery phase now delegates the deep
  codebase scan to the Explore subagent to keep context lean
- install.sh / install.ps1: each command in command/ installs as its own
  ~/.claude/commands/<name>.md (legacy bundle backed up on upgrade)
- skill/SKILL.md, CLAUDE.md, README.md, CHANGELOG.md: docs updated for
  the plugin install path, sync command, and new line cap

Verified via smoke test: validator constants, template targets, generator
output line counts across 5 presets (all <= 150), context files with
backlinks, @-import chain in modular root, idempotent merge_with_existing,
validator status transitions at the new cap, analyzer quality differential,
and plugin manifest JSON shape with all referenced paths existing on disk.

skill/validator.py

@@ -12,9 +12,10 @@ import re
 class BestPracticesValidator:
     """Validates CLAUDE.md files against best practices and guidelines."""
 
-    # Maximum recommended line count
-    MAX_RECOMMENDED_LINES = 300
-    WARNING_THRESHOLD_LINES = 200
+    # Hard cap: every CLAUDE.md (root or modular) must stay under this.
+    # Modular split is required when content would exceed this cap.
+    MAX_RECOMMENDED_LINES = 150
+    WARNING_THRESHOLD_LINES = 120
 
     # Minimum content requirements
     MIN_LINES = 20