🚑 fix(fb-31): apply final review nits

- kebab-app: add #[doc(hidden)] to ingest_stdin_with_config (CLAUDE.md
  convention — all *_with_config functions should have this attribute;
  fb-31's first impl missed it on the second facade fn).
- SKILL.md: "Since v0.4.0" → "Since v0.3.1" (MCP shipped in fb-30
  release v0.3.1; the wrong version claim was introduced in fb-30 doc
  sync and carried forward into fb-31).
- tools_call_ingest_file: add idempotency test (second call with same
  content → unchanged=1, new=0). Spec called for two tests; first impl
  shipped only the happy path.

Version bump 0.3.1 → 0.3.2 deferred to separate `chore/bump-v0.3.2` PR
mirroring fb-27 + fb-30 precedent (commits 73f5d73 / 5495d96).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

integrations/claude-code/kebab/SKILL.md

@@ -73,7 +73,7 @@ If a call fails or returns suspicious output, run `kebab doctor` first — it su
 
 ## MCP server (recommended over CLI subprocess wrapping)
 
-Since v0.4.0, `kebab` exposes an MCP (Model Context Protocol) stdio server. Configure once in `~/.claude/mcp.json`:
+Since v0.3.1, `kebab` exposes an MCP (Model Context Protocol) stdio server. Configure once in `~/.claude/mcp.json`:
 
 ```json
 {