feat(gitflow): auto-purge transient superpowers artifacts at finish (BDR-065)

_gitflow_purge_transient removes docs/superpowers/{specs,plans} on the
feature/bugfix branch just before the directed merge, so develop's tip
lands clean while the feature commits stay reachable as the archive
(git show <sha>:...). Best-effort: never aborts a finish (no-op when
absent, skip on dirty paths, restore index+tree on commit failure).
Opt-out GITFLOW_PURGE_TRANSIENT=0; purge-transient CLI verb. Automates
the manual post-merge cleanup BDR-065 left as doctrine (slipped once,
655e364). Universal via the ~/.claude/lib symlink. gitflow-test T17 a-d;
shellcheck clean; make test exit 0.

CLAUDE.md

@@ -32,8 +32,13 @@ or re-run `make plugin`.
 
 `docs/superpowers/specs/**` and `docs/superpowers/plans/**` are run-time
 artifacts of a feature pipeline (subagent briefs, reviewer references).
-They are committed DURING the run and DELETED in the post-merge cleanup
-(BDR-065) — git history at the feature commits is their archive. Durable
-knowledge goes to `.claude/memory/` registries, never to these files.
-Derived scan/audit outputs (`.audit/**`) are gitignored and never
-committed, even redacted (LRN-124).
+They are committed DURING the run (the SDD worktree + reviewers read them
+from disk — NOT gitignored), then AUTO-PURGED by `gitflow finish` on a
+`feature`/`bugfix` branch, before the merge, so develop's tip stays clean
+(BDR-065, `lib/gitflow.sh` `_gitflow_purge_transient`). The feature commits
+stay reachable from develop, so `git show <sha>:docs/…` is still the archive.
+Opt out with `GITFLOW_PURGE_TRANSIENT=0`. NOT in scope: `.claude/tasks/{contracts,plans}`
+(durable, versioned, referenced by decisions.md). Durable knowledge goes to
+`.claude/memory/` registries, never to these files. Derived scan/audit
+outputs (`.audit/**`) are gitignored and never committed, even redacted
+(LRN-124).