Canonicalize 21-hook pre-commit template for adoption across production repos

[avrabe]

Part of the V&V coverage initiative.

Problem

rivet ships a 21-hook .pre-commit-config.yaml that is the reference for the workspace. Other production repos have drifted — loom has 12 hooks, meld 7, spar 7, gale only 3; relay and sigil have no pre-commit config at all. Divergence is noise that weakens the assessor story.

Acceptance

• Publish the canonical template as a versioned, copyable asset (e.g. rivet/templates/pre-commit/.pre-commit-config.yaml)
• Document the rationale per-hook in docs/pre-commit.md: why each hook matters, which standard clause it helps satisfy
• Define tiers (optional but recommended):
  • Tier 1 (baseline): 11 hooks — formatting, file hygiene, cargo-fmt/clippy/test, rivet-validate
  • Tier 2 (safety-critical): + cargo-audit, cargo-deny, cargo-bench-check
  • Tier 3 (verification-heavy): + cargo-mutants
• Open adoption follow-up issues on repos that need alignment: kiln, loom, meld, relay, sigil, synth, spar, gale, wohl
• Rivet CI gate that verifies adopter repos are at their declared tier (optional; stretch goal)

Notes

• Rust versions in hooks must match rust-toolchain in each adopter; provide a variable-substitution pattern
• cargo-mutants is slow; Tier 3 can use stages: [manual] to keep interactive dev fast
• Document the opt-outs that are acceptable vs unacceptable per tier

[avrabe]

Draft PR up: #222.

Covers the in-scope deliverables (canonical template at templates/pre-commit/.pre-commit-config.yaml, per-hook rationale doc at docs/pre-commit.md, advisory tier annotations). Two open decisions called out in the PR body:

1. Tier partition — the issue's "T1=11, T2=+3, T3=+1" totals 15; the template covers all 21 hooks under a defensible reading and the doc flags the gap explicitly.
2. cargo +stable pinning vs. variable-substitution placeholder for rust-toolchain.toml.

Out of scope and needs to be filed manually: adoption follow-up issues on kiln, loom, meld, relay, sigil, synth, spar, gale, wohl. Automated agents in this workflow are scoped to pulseengine/rivet, so cross-repo issue creation has to come from a session with broader scope.

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