docs(pre-commit): canonical 21-hook template + tier docs (#186)

[avrabe]

Refs: #186

Summary

First in-scope cut at the canonical pre-commit template story. Two new files:

• templates/pre-commit/.pre-commit-config.yaml — copy-pasteable source of truth, mirroring rivet's own 21-hook config but with CUSTOMIZE: markers for per-project knobs.
• docs/pre-commit.md — per-hook rationale table, standard-clause mapping, advisory T1 / T2 / T3 tier system, adoption recipe, drift policy.

Acceptance-criteria mapping (from #186)

AC bullet	Status here
Publish canonical template as versioned, copyable asset	Done — templates/pre-commit/.pre-commit-config.yaml
Document rationale per-hook + standard clause	Done — docs/pre-commit.md per-hook table
Define T1 / T2 / T3 tiers	Proposal only — tier labels included as inline annotations, doc explicitly notes the partition is open for review
Open adoption follow-up issues on kiln/loom/meld/relay/sigil/synth/spar/gale/wohl	Out of scope — automated GitHub access is restricted to pulseengine/rivet; needs a session with broader org scope (or manual filing)
CI gate that verifies adopter tier compliance	Stretch goal; not addressed

Why draft

Two open decisions need maintainer review before this leaves draft:

1. Tier partition. The 21-hook → T1 / T2 / T3 split in Canonicalize 21-hook pre-commit template for adoption across production repos #186 totals 15 hooks (T1=11 + T2=3 + T3=1), leaving 6 of the file-hygiene hooks unassigned. The template assigns all 21 to a tier under a defensible reading; the doc flags this explicitly. Confirm or adjust.
2. cargo +stable pinning. Hard-coded for the template; if adopters should template-substitute the channel from their rust-toolchain.toml instead, say the word and I'll switch the example to a placeholder.

Test plan

• Template parses as valid YAML; hook count = 21 (matches issue claim)
• No artifact YAML touched, so rivet validate is unaffected
• CI green
• Maintainer confirms tier assignment
• Cross-repo adoption issues filed separately (see "Out of scope" above)

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