Trim PUB GATE — delegate consumer-grep to zlint unused-decls

[indykish]

Summary

• Mechanical consumer-grep delegated to zlint's unused-decls: error rule. The PUB GATE body keeps the agent-only design call: shape verdict + no inheritance.
• Per-edit proof block compresses from "consumers + shape + no-inheritance" → "shape verdict + no-inheritance". Roughly half the ceremony per new pub surface.
• unused-decls: error is now marked load-bearing in docs/ZIG_RULES.md so a future ruleset trim doesn't silently bypass the gate.

Why

zlint already catches "pub fn with no consumer" mechanically — every make lint run fails on it. The gate's pre-edit consumer-grep loop was paying the same cost twice. What zlint can't decide:

• Whether a new public symbol belongs on a struct as a method or as a free fn (file-as-struct vs conventional layout).
• "I cloned the sibling's justification verbatim instead of doing my own grep + verdict" — a reasoning anti-pattern, not a syntactic one.

So the gate body retains those two design calls; the mechanical orphan-pub check moves to lint.

Changes

• AGENTS.md row 10 — gate description trimmed; consumer-grep delegation made explicit.
• docs/gates/pub-surface.md — pre-edit step 3 (consumer-grep loop) and step 4 (manual orphan sweep) replaced with "own shape verdict per new surface, no inheritance"; required output drops the per-symbol consumer= field; self-audit calls out the load-bearing make lint run.
• AGENTS_INVARIANCE.md Q4.4 + Q4.4a — rephrased to match new contract; Q4.4b (per-edit proof-line) and Q4.4c (threshold-as-floor) unchanged.
• docs/ZIG_RULES.md ZLint Policy — unused-decls: error marked load-bearing with a cross-reference to PUB GATE.

Audit

✅ ALL CHECKS PASSED (17/17)
size 25548 bytes (limit 25600) — 52 bytes under
pre-push: ✅ AGENTS_INVARIANCE sign-off PASS for 00e426f (age 0 min)

Test plan

• bash scripts/audit-agents-md.sh → ALL CHECKS PASSED
• Pre-commit hook runs audit + commits
• Pre-push hook validates fresh .agents-invariance-signoff
• First real consumer of the new format: usezombie M68 §10a code commit (next PR) cites the trimmed PUB GATE proof
• Project repos refresh AGENTS.md via bin/sync-agents once merged

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