chore: gitignore internal planning docs (plans/ and specs/)

Move docs/superpowers/plans/ and docs/superpowers/specs/ to local-only.
These document in-progress work for future-Claude context and are
not meaningful to external consumers.

docs/superpowers/reviews/ deliberately stays tracked — the security
reviews there are a community trust signal, explicitly referenced
from CLAUDE.md "Security & reviews" section.

Historical note: 8 files (3 plans + 5 specs from 2026-04-16/17)
remain in git history on origin/dev from prior commits. Removing
them from history would require a destructive force-push on a
shared branch, which costs more than keeping them archived. The
going-forward hygiene (no new internal docs on public remote) is
what matters.

Files removed from index (preserved locally on disk):
- 3 plans from 2026-04-16/17 (filter-exclusion, drift, interface-parameters)
- 5 specs from 2026-04-16/17 (filter-exclusion, drift, interface params, enum null-clearing, filter-exclusion rollout)

Author: ctrl-alt-automate

.gitignore

@@ -45,3 +45,11 @@ TestResults/
 # - Test-ExeDevVMs.ps1
 # - scripts/setup-exe-dev-environment.sh
 # - .github/workflows/exe-dev-tests.yml
+
+# Internal planning and design artifacts (local-only)
+# These document in-progress work for future-Claude context but are
+# not intended for public consumption. docs/superpowers/reviews/ is
+# deliberately excluded from this ignore — security reviews remain
+# public as a community trust signal (see CLAUDE.md "Security & reviews").
+docs/superpowers/plans/
+docs/superpowers/specs/