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).

specify/SKILL.md

@@ -270,8 +270,11 @@ it as a target, not verified fact.
 
 Path: under `docs/specs`, named `slug.md` per the fixed naming
 convention (see `docs/conventions.md` § Naming) — a short kebab-case
-topic slug, no numeric prefix. The directory is git-ignored; the spec is
-created there and never `git add`'d.
+topic slug, no numeric prefix. The directory is git-ignored via the
+project's committed `docs/.gitignore`; if that file does not yet ignore
+`specs/`, create it first (`docs/conventions.md` § File layout) so the
+spec can never be committable. The spec is created under `docs/specs` and
+never `git add`'d.
 
 Structure: