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.

settings.json

@@ -292,7 +292,17 @@
       "Bash(* | sh)",
       "Bash(* | sh -*)",
       "Bash(* | sudo *)",
-      "Bash(chattr *)"
+      "Bash(chattr *)",
+      "Bash(GIT_CONFIG_GLOBAL=*)",
+      "Bash(GIT_CONFIG_SYSTEM=*)",
+      "Bash(GIT_CONFIG=*)",
+      "Bash(env GIT_CONFIG*)",
+      "Bash(git config --unset core.hooksPath*)",
+      "Bash(git config --unset-all core.hooksPath*)",
+      "Bash(git config --local core.hooksPath *)",
+      "Bash(git config gitflow.*)",
+      "Bash(git config --global gitflow.*)",
+      "Bash(git config --local gitflow.*)"
     ],
     "ask": [
       "Bash(bash -c *)",