feat(profile): add gstack on|off verb to lib/profile.sh

Centralize gstack toggling in the `profile` command without losing the
active-profile label.

  - `gstack on`  re-enables ALL parked gstack skills (moves
    skills-disabled/gstack__* back) but does NOT touch .active-profile,
    so the user layers full gstack on top of their current profile and
    the statusline label is preserved. Unlike `reset`, which clears the
    label to "none".
  - `gstack off` disables gstack skills not listed in the active profile;
    errors cleanly when no profile is active (needs one to know what to
    keep).

Refactor (behavior-preserving): extract three shared helpers
`enable_all_gstack`, `disable_gstack_not_in`, `parked_gstack_count` and
rewire `cmd_reset` + `cmd_set` to reuse them instead of duplicating the
symlink-toggle loops. Wire `gstack` into main() dispatch, usage(), and the
header usage block.

Docs: SKILL.md argument-hint, examples, and output-policy updated. The
generic `make profile cmd="gstack on"` target already covers Make usage.

Verified: shellcheck CLEAN, `bash -n` OK, 6-case test (help, bad-action,
off-with-no-profile, on, off-trim, on-cycle) with final assertion that the
live symlink state was restored exactly to its pre-test value.

Memory: capitalize BDR-018 (decision), LRN-024 (DRY helper-extraction
pattern), BLK-007 (6 gstack source skills ios-*/spec unlinked post
submodule bump — open follow-up), EVAL-002 (self-eval, false "full.profile
bug" flag corrected pre-edit). Backfill index drift: BDR-017, BLK-005/006.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

skills/profile/SKILL.md

@@ -8,7 +8,7 @@ description: |
   "switch to design", "set profile", "active profile", "quel profil",
   "profil design", "active les skills design", "désactive gstack",
   "réduire le bruit gstack".
-argument-hint: list | show <name> | current | apply <name> | set <name> | reset | diff <a> <b>
+argument-hint: list | show <name> | current | apply <name> | set <name> | reset | gstack on|off | diff <a> <b>
 disable-model-invocation: false
 allowed-tools:
   - Bash
@@ -81,9 +81,13 @@ bash "$HOME/.claude/lib/profile.sh" apply <name>
 # Enable only skills in profile (disables non-listed gstack skills)
 bash "$HOME/.claude/lib/profile.sh" set <name>
 
-# Re-enable every gstack skill (undo any set/apply)
+# Re-enable every gstack skill (undo any set/apply) — resets active label to "none"
 bash "$HOME/.claude/lib/profile.sh" reset
 
+# Toggle gstack only, keeping the active-profile label intact
+bash "$HOME/.claude/lib/profile.sh" gstack on    # re-enable ALL gstack on top of current profile
+bash "$HOME/.claude/lib/profile.sh" gstack off   # disable gstack skills not in the active profile
+
 # Compare two profiles
 bash "$HOME/.claude/lib/profile.sh" diff <a> <b>
 ```
@@ -101,9 +105,9 @@ bash "$HOME/.claude/lib/profile.sh" $ARGUMENTS
 
 ## Output policy
 
-- After `set` / `apply` / `reset`: show the count of skills moved + tell the
-  user to start a new Claude session to pick up the changes (Claude scans
-  `skills/` at session start).
+- After `set` / `apply` / `reset` / `gstack on|off`: show the count of skills
+  moved + tell the user to start a new Claude session to pick up the changes
+  (Claude scans `skills/` at session start).
 - After `current`: report the active profile + match percentage.
 - After `show`: render the table directly — no extra commentary unless the user
   asks.