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).

docs/conventions.md

@@ -28,13 +28,17 @@ elsewhere is the rare exception and states the override in its
 `CLAUDE.md` project facts.
 
 Both directories are **git-ignored** — their contents are transient
-working files, never committed (see § Lifecycle). The plugin ignores
-them **globally**: `docs/specs/` and `docs/plans/` live in the user's
-global git excludes (`~/.config/git/ignore`, the XDG default that
-applies to every repo), next to the other plugin-wide ignores. One
-entry set covers every project — no per-project `.gitignore` line is
-needed, though a project MAY add its own if it wants the ignore to
-travel with the repo.
+working files, never committed (see § Lifecycle). The ignore is
+**project-local**: each project commits a `docs/.gitignore` whose two
+entries, `specs/` and `plans/`, are relative to `docs/` and so match
+`docs/specs/` and `docs/plans/`. That `docs/.gitignore` is the one
+committed thing — it travels with the repo; the spec and plan files it
+covers never are. It is created once (whichever skill first writes a
+working file — `specify`, or `planner` on a direct tidy dispatch —
+creates it if absent) and committed like any other repo file, after
+which both directories are invisible to git for every future cycle. A
+new project sets this up as part of adopting the plugin
+(`../templates/CLAUDE.md.fragment`).
 
 ## Lifecycle