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/post-merge

@@ -4,6 +4,8 @@
 # holds. Never fails the commit: no origin / offline / refused → warning only.
 # Opt out for one command with GITFLOW_NO_PUSH=1 (throwaway repos, tests).
 [ "${GITFLOW_NO_PUSH:-0}" = 1 ] && exit 0
+# Per-repo opt-out (no push rights on a foreign clone): git config gitflow.autopush false
+[ "$(git config --bool --default true gitflow.autopush)" = false ] && exit 0
 git remote get-url origin >/dev/null 2>&1 || exit 0
 br=$(git symbolic-ref --short -q HEAD 2>/dev/null) || exit 0   # detached HEAD — nothing to track
 if command -v timeout >/dev/null 2>&1; then t="timeout ${GITFLOW_PUSH_TIMEOUT:-30}"; else t=""; fi