feat(profile): default profile = full; reset applies it, current is label-driven

No profile selected (.active-profile absent, empty or legacy "none") now
means the `full` profile is in force: DEFAULT_PROFILE declared once in
lib/profile.sh, resolved by active_profile(); the statusline reads the
constant and shows `full` instead of `?`; `gstack off` trims to it instead
of erroring. `reset` goes to the default profile (= `set full`: enables its
list, parks any non-listed gstack or managed item). `current` names the
active label and scores that profile only, saying `default — not applied
yet` until a set/apply/reset wrote the cache; the "none" sentinel and the
cross-profile best-guess scan are gone (they keyed on the parked-gstack
count, which says nothing under BDR-030's gstack-off default). Hermetic
suite lib/tests/profile-default.test.sh (29 checks) seeds gstack as OFF like
a real tree. Citers updated: profile SKILL, Makefile help, plugin-advisor
PROFILE line + reset paragraph, toggle-external header.

lib/toggle-external.sh

@@ -26,7 +26,7 @@
 # audit skills, etc.) instead of all-or-nothing gstack toggling, use:
 #   bash lib/profile.sh list
 #   bash lib/profile.sh set <design|dev|qa|audit|minimal>
-#   bash lib/profile.sh reset
+#   bash lib/profile.sh reset       # back to the default profile (full)
 # ============================================================
 set -euo pipefail