fix: AI mode generates unwanted commit changelog in README_AI.md

[dreamlx]

Problem

When using codeindex scan --ai, the AI (LLM) spontaneously appends commit changelog sections to the generated README_AI.md, even though this is not part of the design intent.

Example: src/codeindex/README_AI.md contains ~60% noise from empty commit entries (lines 126-262):

**Commit ``**: 

Changed files:
- `smart_writer.py`

**Commit ``**: 

Changed files:
- `parser.py`

Root Cause

• SmartWriter's structural generators (overview/navigation/detailed) do not include any git log/commit logic
• The commit information is added by the LLM during AI-mode generation — it over-interprets the prompt and adds changelog content on its own
• This is confirmed by comparing:
  • src/README_AI.md (structural mode, (navigation) tag) → clean, no commit info
  • src/codeindex/README_AI.md (AI mode, no mode tag) → polluted with commit changelog

Impact

• Wastes ~60% of README_AI.md token budget on noise
• Commit info is redundant — git log already provides this
• Degrades AI agent experience when reading README_AI.md for code understanding

Expected Action

Add explicit constraints in the AI prompt to prevent LLM from generating commit history, changelog, or "Recent Changes" sections. The prompt should instruct:

• Only generate content based on the provided ParseResult (symbols, imports, structure)
• Do NOT add git commit history, changelog, or recent changes sections
• Do NOT fabricate information not present in the input data

Discovered During

Dogfooding / brainstorm session on README_AI.md organizational structure (2026-03-12).

[dreamlx]

Fixed in v0.23.0. Post-commit hook redesigned as thin wrapper delegating to codeindex hooks run post-commit (Python). No more custom AI prompts injecting git diff — uses standard codeindex scan pipeline. Run codeindex hooks install post-commit --force to upgrade existing hooks.