feat(skills): blank-slate skills — install --no-skills + opt-out/opt-in (#36228)

* feat(install): --no-skills flag for blank-slate default profile

Add an install-time --no-skills flag so the default ~/.hermes profile can
be created with zero bundled skills, matching what
`hermes profile create --no-skills` already does for named profiles.

The flag writes $HERMES_HOME/.no-bundled-skills and skips the install-time
seed. sync_skills() now honors that marker with an early return
(skipped_opt_out=True), so neither the installer, a later `hermes update`,
nor a direct sync re-injects bundled skills into a profile that opted out.

Previously the marker was only checked by seed_profile_skills() (named
profiles); the default profile had no opt-out and `hermes update` would
re-seed it every time.

Tests: TestNoBundledSkillsOptOut covers marker-present (no-op) and
marker-absent (normal seed) paths.

* feat(skills): hermes skills opt-out / opt-in for existing profiles

Adds an interactive counterpart to the install-time --no-skills flag so
an already-installed profile (default or named) can toggle the
.no-bundled-skills marker without reinstalling.

- `hermes skills opt-out` writes the marker (stop future seeding). Safe
  by default: nothing on disk is touched.
- `hermes skills opt-out --remove` ALSO deletes already-present bundled
  skills, but ONLY ones that are manifest-tracked AND byte-identical to
  their origin hash. User-edited bundled skills, hub-installed skills, and
  hand-written skills are never removed. Previews + confirms before
  deleting (--yes to skip).
- `hermes skills opt-in [--sync]` removes the marker and optionally
  re-seeds immediately.

Core logic lives in tools/skills_sync.py (set_bundled_skills_opt_out,
is_bundled_skills_opt_out, remove_pristine_bundled_skills) reusing the
existing manifest origin-hash machinery for the safety check.

Tests: TestOptOutToggleAndRemove covers marker toggle idempotency and
proves user-modified + non-bundled skills survive --remove.

* docs: blank-slate skills — install --no-skills + opt-out/opt-in

- features/skills.md: new 'Starting with a blank slate' section covering
  the install flag, profile-create flag, and runtime opt-out/opt-in, with
  a safe-by-default note.
- reference/cli-commands.md: document the new skills opt-out / opt-in
  subcommands + examples.
- reference/profile-commands.md: fix the marker filename (was .no-skills,
  actually .no-bundled-skills) and cross-link the runtime commands.

Validated with a full docusaurus build (exit 0); the three edited pages
compile clean with no new warnings.

hermes_cli/main.py

@@ -13267,6 +13267,43 @@ Examples:
         help="Skip confirmation prompt when using --restore",
     )
 
+    skills_opt_out = skills_subparsers.add_parser(
+        "opt-out",
+        help="Stop bundled skills from being seeded into this profile",
+        description=(
+            "Write the .no-bundled-skills marker so the installer, "
+            "`hermes update`, and any direct sync stop seeding bundled skills "
+            "into the active profile. By default nothing already on disk is "
+            "touched. Pass --remove to ALSO delete bundled skills that are "
+            "unmodified (user-edited and hub/local skills are never removed)."
+        ),
+    )
+    skills_opt_out.add_argument(
+        "--remove",
+        action="store_true",
+        help="Also delete already-present unmodified bundled skills",
+    )
+    skills_opt_out.add_argument(
+        "--yes",
+        "-y",
+        action="store_true",
+        help="Skip confirmation prompt when using --remove",
+    )
+
+    skills_opt_in = skills_subparsers.add_parser(
+        "opt-in",
+        help="Re-enable bundled-skill seeding (undo opt-out)",
+        description=(
+            "Remove the .no-bundled-skills marker so bundled skills are seeded "
+            "again on the next `hermes update`. Pass --sync to re-seed now."
+        ),
+    )
+    skills_opt_in.add_argument(
+        "--sync",
+        action="store_true",
+        help="Re-seed bundled skills immediately instead of waiting for update",
+    )
+
     skills_repair_official = skills_subparsers.add_parser(
         "repair-official",
         help="Backfill or restore official optional skills from repo source",