refactor(pipeline): ignore specs/plans via project-local docs/.gitignore

Replace the global ~/.config/git/ignore mechanism with a committed,
per-project docs/.gitignore whose entries `specs/` and `plans/` are
relative to docs/. The ignore now travels with each repo instead of
living in the user's global excludes.

specify (Step 3) and planner (intro) create docs/.gitignore on demand
if a project has not set it up yet, so a spec/plan can never become
committable. conventions.md § File layout is the single source; the
onboarding fragment tells a new project to set it up.

The global ~/.config/git/ignore block is removed separately (it is
outside this repo).

planner/SKILL.md

@@ -17,8 +17,11 @@ into bites small enough that a subagent can execute one in
 
 Plans live under `docs/plans` with the fixed name shape
 `slug.md` — a short kebab-case topic slug, no numeric prefix (see
-`docs/conventions.md` § Naming). `docs/plans` is git-ignored; the plan
-is a working file, never committed. The header references the parent spec. The plan
+`docs/conventions.md` § Naming). `docs/plans` is git-ignored via the
+project's committed `docs/.gitignore`; on a direct tidy dispatch that
+skipped `specify`, create that file first if it does not yet ignore
+`plans/` (`docs/conventions.md` § File layout). The plan is a working
+file, never committed. The header references the parent spec. The plan
 decomposes work into tasks; each task is the unit of review
 (spec-compliance and quality gates inside `implement`), not
 the unit of commit. Commits are an orchestrator-only decision