fix(gitflow): every autopush reader fails closed and names an invalid value

Run D1 of manual-push mode (BDR-114). `git config --bool --default true
gitflow.autopush` only covered a MISSING key: an unparseable value made
git die with empty output, the `= false` test failed, and every push ran
again. A typo on a work machine silently re-enabled the pushes it was
meant to stop.

- lib/gitflow.sh: `_gitflow_push_off` reads the mode through the lib
  verb (`push-mode`); anything but `auto` is push-off, and the verb's
  stderr line names an invalid value during start/finish.
- Emitted post-commit/post-merge hooks (POSIX sh, standalone): push only
  when the key reads `true` or is unset; `false` exits quietly; any
  other result prints one stderr line ("NOT pushed, treated as manual
  push mode") and exits 0. Mirrors gitflow_push_mode.
- .githooks/ and githooks/ regenerated files-only through `emit-hook`
  (no config read or write; .git/config hash unchanged).
- hooks/unpushed-guard.sh: mode from the lib verb (absolute lib path
  resolved before any cd, no temp file); anything but auto is manual;
  the SessionStart line names an invalid or unreadable value.
- Tests: gitflow-test T18q block (invalid → start, hook and finish push
  nothing and say so; `true` → the hook pushes; emitted hook is
  POSIX-clean), unpushed-guard T14 rewritten.

lib/tests/unpushed-guard.test.sh

@@ -66,8 +66,10 @@ git config gitflow.autopush flase
 echo i>i; git add i; git commit -q -m i
 out=$(fire SessionStart "$PWD")
 check T14-invalid-named "$(has "$out" "not a boolean")" yes
-check T14-invalid-treated-auto "$(has "$out" "unpushed work")" yes
-check T14-invalid-stop-auto "$(has "$(fire Stop "$PWD")" "1 commit(s)")" yes
+check T14-invalid-prefix "$(has "$out" "ℹ manual push mode:")" yes
+check T14-invalid-treated "$(has "$out" "treated as manual")" yes
+check T14-invalid-no-warn "$(has "$out" "unpushed work")" no
+check T14-invalid-stop-silent "$(fire Stop "$PWD")" silent
 git config --unset gitflow.autopush
 check T15-unset-auto-intact "$(has "$(fire Stop "$PWD")" "1 commit(s)")" yes
 git config gitflow.autopush false; git remote remove origin