feat(test-deletion-guardrail): block unjustified test removal in tdd-cycle + verify

[MountainUnicorn]

Summary

Implements specs/test-deletion-guardrail.md (v0.9.0, M3 pre-GA hardening). Defends ADD's signature TDD claim against the Kent Beck / TDAD-paper failure mode: coding agents prefer to delete the failing test rather than satisfy it, which silently corrupts the RED → GREEN separation.

• scripts/check-test-count.py — stdlib-only snapshot / compare / gate CLI. One-shot --baseline <ref> form is the entry point invoked by /add:verify Gate 3.5.
• core/lib/impact-hint.sh — pure shell + jq + grep "files-likely-affected" hint fed to /add:implementer at start of GREEN.
• core/knowledge/test-discovery-patterns.json — regex catalog covering Python, TS/JS, Go, Ruby, Rust (extensible per project).
• New Gate 3.5 "Test Surface Integrity" in /add:verify, between Gate 3 and Gate 4.
• core/rules/tdd-enforcement.md gains the "Test-Deletion Invariant" section (AC-028).
• scripts/compile.py now ships core/lib/ + core/knowledge/ into both runtimes.

Acceptance Criteria

• AC-001..AC-006 — RED snapshot + fail-on-empty + commit convention
• AC-007..AC-013 — GREEN re-snapshot + comparison (added/removed/renamed/replaced) + normalized body hashing
• AC-014..AC-017 — Gate 3.5 placement, missing-snapshot error, structured summary
• AC-018..AC-024 — Files-likely-affected hint (diff + import resolution + spec path scan + anti-pattern learnings lookup + empty-diff fallback)
• AC-025..AC-027 — Telemetry JSONL (deferred to companion telemetry-jsonl spec; integration contract documented in this PR)
• AC-028 — Rule invariant

Justification markers accepted by the gate:

1. Commit trailer in range base..HEAD: [ADD-TEST-DELETE: <reason>]
2. Per-cycle override file: .add/cycles/cycle-{N}/overrides.json with kind: "test-rewrite"

Test plan

• 15 fixture tests in tests/test-deletion-guardrail/ covering the gate matrix: count-same, count-increased, count-decreased (no override → FAIL), count-decreased-with-approval, empty-before (bootstrap), rename, replacement-without-approval, replacement-with-approval, missing-green, compare-json-shape, rename-detection, compare-summary-format, discovery-python-basic, empty-red-fails, impact-hint-basic
• python3 scripts/compile.py --check passes (no drift)
• python3 scripts/validate-frontmatter.py passes
• bash tests/hooks/test-filter-learnings.sh passes (regression check)
• Dog-food: python3 scripts/check-test-count.py --baseline origin/main returns PASS (net test count increased from 0 to 0 in harness terms; no removals)

Part of

M3 pre-GA hardening (Cycle 3). Spec: specs/test-deletion-guardrail.md. Complements telemetry-jsonl (Cycle 3) which owns the JSONL emitter referenced by AC-025..AC-027.

Files Touched

Path	Change
core/skills/tdd-cycle/SKILL.md	§RED snapshot, §GREEN snapshot, §--allow-test-rewrite, Gate 3.5 reference
core/skills/verify/SKILL.md	New Gate 3.5 between Gate 3 and Gate 4
core/skills/test-writer/SKILL.md	Step 4b: capture + commit RED snapshot
core/skills/implementer/SKILL.md	Pre-flight 1b (impact hint), Step 5b (no-delete directive), Step 7 (GREEN snapshot)
core/rules/tdd-enforcement.md	New "Test-Deletion Invariant" section
core/knowledge/test-discovery-patterns.json	New language regex catalog
core/lib/impact-hint.sh	New shell helper
scripts/check-test-count.py	New snapshot/compare/gate CLI
scripts/compile.py	Copy core/lib/ + core/knowledge/ into both runtimes
tests/test-deletion-guardrail/	New fixture test suite + runner
docs/plans/test-deletion-guardrail-plan.md	Implementation plan + AC coverage matrix
CHANGELOG.md	Entry under [Unreleased]

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