feat(feature-planner): add pre-implementation plan reviewer + review hardening

Add a super-intensive, adversarial plan-reviewer agent that runs after the plan
is drafted and BEFORE the approval gate, to catch wasteful or wrong design
before any implementation effort is spent. Verdict can be REDESIGN REQUIRED;
findings are sorted into Critical Gaps (block) vs Suggestions (non-blocking) so
the two never get conflated at the approval gate. It must open the codebase to
verify claims (files exist, "no existing impl" holds, "parallel" phases are
disjoint). Conditional trigger: 4+ phases / Orchestrator / Automated /
security|data-sensitive. Wired into SKILL.md, discovery-and-planning (Step 5.5),
agent-spawning (spawn pattern + opus model profile + cost), and
modes-and-strategies (most valuable in Automated mode, the only human gate).

Review hardening also addressed:
- test-writer derives assertions from acceptance criteria, not the diff, so a
  failing test is signal (impl bug) rather than something to "fix" to match
  buggy behavior
- early per-phase security/data pass for sensitive phases (not only final review)
- Outstanding Warnings ledger in PLAN.md so non-blocking warnings can't
  accumulate invisibly across phases
- Automated-mode global circuit breaker (cost / agent-count cap)
- new `data` domain tag carrying a migration-safety addendum

Bump README Last updated date.

README.md

@@ -1,6 +1,6 @@
 # Claude Code Facts
 
-> Last updated: 2026-05-26
+> Last updated: 2026-06-01
 
 A collection of interesting and useful facts, tips, tools, and guides for working with Claude and Claude Code.