feat(agent): merge grok ceremony lines into the one residual

[TheGreatAxios]

Summary

• Adds a minimal self-contained promptResidual(family) hook in src/agent/model-family-policy.ts (grok-only; no GLM row per the <30min rule) and merges the three CL-7768 ceremony lines (no git, no pre-plan, verify once) into the single grok residual.
• buildGrokLeafAntiThrashNote now composes promptResidual("grok") verbatim: one residual, no line twice, don't re-read appears exactly once. No GPT/Claude rows or package extraction touched.

Verification

• RED: ec02a7e (new src/agent/grok-residual.test.ts failed: promptResidual not found).
• GREEN: 0e7935f — bun test src/agent/grok-residual.test.ts: 5 pass, 0 fail; related suites (agent/prompts, agent/model-family-policy, subagent/provider-family, prompts): 91 pass, 0 fail; bun run typecheck: exit 0; pre-commit typecheck+build: ok.

Dependency note

• Blocked by / depends on sibling CL-8297 (canonical promptResidual hook not yet merged): this branch carries a minimal local hook so it is green standalone. On CL-8297 landing, rebase and reconcile to the canonical hook (drop the local copy, keep the merged grok text once). Shared library CL-8269 packages/prompt-variance should consume this residual, not invent a second Grok block.

Fixes CL-8296

[linear-code]

CL-8296