feat(graphify): upgrade to 0.8.13 — gemini backend, encoding, monorepo, CLI export

Major SKILL.md rewrite:
- Fast path: skip extraction when graph exists and user asks a question
- Gemini backend replaces Kimi as default external LLM
- All file I/O uses ensure_ascii=False + encoding="utf-8"
- Monorepo support via per-subfolder extraction + merge
- Obsidian/HTML export via CLI instead of inline Python
- Node ID format includes parent dir to prevent ghost duplicates
- file_type gains "concept" as valid value
- Subagent chunk paths must be absolute
- --help flag prints usage and stops
- Large corpus gate raised from 200 to 500 files

Bumps gstack submodule to 026751e.

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

skills/graphify/.graphify_version

@@ -1 +1 @@
-0.7.0
+0.8.13