docs(skills): replace metaphor labels with plain language

[chipfighter]

What

Replace metaphor-style labels in skill bodies with plain language. A label like "gate" / "iron" / "red line" makes the reader decode an image first; the plain word says the same thing directly, for humans and models alike.

before	after	files
Hard gate:	Hard rule:	brainstorming
Iron law: / Iron rule:	Hard rule:	tdd, systematic-debugging, spec-sync
Iron sequence:	Always in this order:	git-flow
## Gate (…) / pass the gate above	## Required checks (…) / pass the checks above	verify-before-done
## Red lines	## Hard rules	spec-sync, verify-before-done

Side effect: all five rule-opener lines now share one uniform label (Hard rule:).

Checked and deliberately kept

• Frontmatter descriptions and the SessionStart primer were already metaphor-free — the frozen 4944-character fixed-context baseline is untouched.
• Standard technical terms stay: guardrails, source of truth, red-green-refactor, seed/skeleton, code freeze, green (CI), release gate (CLAUDE.md, standard CI term).
• Rule strength is unchanged: labels got plainer, not softer.
• No version bump; manifests stay 0.8.0. Owner decides whether this warrants an 0.8.1 tag.

Verification (all after the edits)

• python tests/test-plugin-metadata.py — passed
• bash tests/test-hooks.sh — passed
• tests/test-windows-hook.ps1 — passed

🤖 Generated with Claude Code