fix(repo): run shared hooks only at pre-commit

[FradSer]

Summary

Limit hooks without an explicit stage to pre-commit. Keep the commit-message validator explicitly on commit-msg.

Why

With both hook types installed, unstaged shared hooks also run at commit-message time. A commit-message validation should not rerun formatting, lint, or staged-file checks. This supplies the missing default stage without changing the explicit validator stage.

Extracted from the unrelated tooling changes removed from #693 at review request 6. Existing issues and PRs were searched; no dedicated open issue was found. No issue is closed by this PR.

Verification

• just fmt-check; just commit-check.
• prek -c config/prek.toml run --stage commit-msg --commit-msg-filename .tmp/COMMIT_EDITMSG: only the commit-message validator runs and passes.
• prek -c config/prek.toml run --stage pre-commit --files config/prek.toml: applicable hooks pass.
• Before/after dry-run comparison confirms shared hooks no longer participate at commit-msg.
• Exact-head just check was attempted on local Linux (Bun 1.4.0): format/docs/lint/type stages passed; the unchanged Driver backpressure fixture failed (openai-app-server-client.test.ts:2055; 2,281 passed / 47 skipped / 1 failed). This initial full-gate failure is retained, not claimed as a pass.
• Fresh hook-stage acceptance passed again. All three shared Driver failure cases pass in isolation; the identical Driver pin also passed the complete #699 hosted gate. No Driver change is included.
• Full hosted CI is triggered by Ready for review; see Checks for this PR’s own current result.

Review

• Closest review area: config/prek.toml.
• Env/config changes: one default hook stage; existing hook installation types remain intact.
• Known trade-offs: contributors must reinstall hooks as documented after hook configuration changes.

Impact

• Generated files / GraphQL / DB / lockfile: N/A; unchanged.
• Risk and rollback: revert this isolated commit. No deployment or migration.

Design

N/A: no product UI changes.