feat(gitflow): hooks in every repo, no per-project step

Global: `make link` generates githooks/ from lib/gitflow.sh and sets git's
global core.hooksPath to ~/.claude/githooks, so every repo on the machine
runs the pre-commit protection and the post-commit / post-merge push, even
one that never ran gitflow init. A repo's own local core.hooksPath still
wins, so hooks/session-start.sh calls `gitflow reconcile-hooks` once per
session and rewrites a .githooks/ that lags the lib (LRN-114 automated);
the pre-commit exemption now covers .githooks/** next to .claude/**.

Per-repo opt-outs for a foreign clone: `git config gitflow.protect false`
(branch model) and `git config gitflow.autopush false` (push). Both, and
the GIT_CONFIG_GLOBAL= / GIT_CONFIG= env bypass, are static deny rules.

`make test` and the two suites that commit on main export
GIT_CONFIG_GLOBAL=/dev/null so the machine's global hooks never fire in
throwaway repos. doctor gains "Git hooks" (global setting, githooks/ equal
to the emitters) and "Scratchpad" (warn when TMPDIR sits on a tmpfs with
usrquota: systemd caps each user at 80% of it, which killed two shells
today, BLK-021). Tests: T18h, T19d, T20 (reconcile), T21 (whitelist and
protect opt-out); this repo's own stale .githooks/ refreshed.

.githooks/pre-commit

@@ -20,17 +20,22 @@ else
   echo "gitflow pre-commit: gitleaks not installed — secret scan skipped (https://github.com/gitleaks/gitleaks)." >&2
 fi
 
+# Per-repo opt-out of the branch model (a clone of a foreign project):
+#   git config gitflow.protect false
+[ "$(git config --bool --default true gitflow.protect)" = false ] && exit 0
+
 case "$br" in
   main|develop) ;;                        # protected — keep checking
   *) exit 0 ;;                                              # working branch — allow
 esac
 
-# whitelist: all-staged-under-.claude/ (memory/doc/deploy helpers) — allow
-if [ -z "$(git diff --cached --name-only | grep -v '^\.claude/' | head -1)" ]; then
+# whitelist: all-staged-under-.claude/ (memory/doc/deploy helpers) or
+# .githooks/ (the hooks themselves, refreshed by the lib) — allow
+if [ -z "$(git diff --cached --name-only | grep -vE '^\.(claude|githooks)/' | head -1)" ]; then
   exit 0
 fi
 
 echo "gitflow pre-commit: BLOCKED — direct commit on '$br'." >&2
 echo "  Branch from the right base (feature/bugfix->develop, hotfix->main), or merge." >&2
-echo "  (.claude/** memory commits are exempt; --no-verify bypasses locally.)" >&2
+echo "  (.claude/** and .githooks/** commits are exempt; foreign clone? git config gitflow.protect false)" >&2
 exit 1