feat(tokens): pr-review-toolkit OFF by default — heaviest plugin, PR-only use

Measured: 6 agent descriptions = ~2.2k tokens injected EVERY session
(the single largest plugin contributor) for a toolkit useful only when
reviewing PRs. enabledPlugins → false; removed from full+backend
profiles (BDR-017 caveat already accepts full excluding rarely-used
items); audit.profile KEEPS it = profile reactivation channel.
Per-PR-session: claude plugin enable pr-review-toolkit@claude-code-plugins
(or bash lib/profile.sh apply audit); a later 'profile set full'
re-disables it via the MANAGED_PLUGINS lifecycle.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016zA3Qh2Q1QpcGXzXxKeDHR

lib/profiles/backend.profile

@@ -34,8 +34,9 @@ guard
 learn
 retro
 
-# Plugin: PR review toolkit (pre-merge audit)
-pr-review-toolkit                 plugin@claude-code-plugins
+# pr-review-toolkit removed (audit 2026-07-02 #12 — ~2.2k tokens, PR-only):
+# enable per PR session via `bash lib/profile.sh apply audit` or
+# claude plugin enable pr-review-toolkit@claude-code-plugins
 
 # CLIs (advisory)
 ctx7                              cli