feat(skills): graft enforcement layer onto discipline skills

[protonspy]

Summary

Adapts the superpowers enforcement technique onto csdd's five discipline skills, while preserving their existing csdd evidence discipline. Each skill gains:

• ## The Iron Law — one inviolable, imperative rule with explicit loophole-closers.
• ## Rationalizations — an | Excuse | Reality | table that pre-empts the exact excuses an agent uses under pressure, ending in the verbatim "Violating the letter of the rule is violating its spirit."
• ## Red Flags — STOP — thought-phrased self-checks that send the agent back to the workflow.

It also tightens each description front-matter to be trigger-only (not a workflow summary, which the superpowers method shows makes agents skip the body), and converts each skill's intro into the required ## Goal section.

Spec

harden-discipline-skills (gitignored layer-3, dogfooded locally) — requirements → design → tasks all human-approved; tasks 1–8 complete. development_flow: unit (prose work; per-behavior "test" is csdd skill validate + byte-parity + subagent pressure test).

What changed (committable)

• internal/templater/templates/skills/{tdd-cycle,verify-change,pr-review,safe-refactor}/SKILL.md.tmpl — the layer-2 shipped baseline, kept byte-identical to the layer-3 copies. csdd-quality-gate is hardened too but is layer-3-only (gitignored), so it's not in this diff.
• internal/cli/update_test.go — sync the test fixture that pinned verify-change's old description (necessary because R4 rewrote it).
• internal/cli/cli_test.go — add the four shipped discipline skills to TestShippedWorkflowSkillsValidate as a regression guard.

Notable findings

• The four shared skills were already failing csdd skill validate (missing ## Goal) before this PR — undetected because no test validated them. Fixed, and the new regression guard locks it in.
• The bundled MCP server (v0.2.6) silently drops development_flow from spec.json when re-serializing (predates that field). Tracked separately as a follow-up issue.

Verification

```

• make check ✅ (gofmt clean + vet + race tests, all packages)
• csdd skill validate (tdd-cycle, verify-change, csdd-quality-gate, pr-review, safe-refactor) ✅ 5/5
• byte-parity layer-2 ↔ layer-3 (4 shared skills) ✅ empty diffs
• subagent pressure test (tdd-cycle, verify-change) ✅ both chose the disciplined option, citing the Iron Law
• code-reviewer (adversarial) ✅ APPROVE, 0 Blockers / 0 Important
• csdd spec test-report ⚠️ 455 passed · 0 failed · 1 skipped (attention: gotestsum not installed; plain go test via make check is green)
  ```

Risks

• Pure prose change to shipped skill templates (+ two test-only edits); no Go production logic touched. Downstream users receive the hardened skills on the next csdd init.

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