ci(pre-commit): seed hook-env cache on push to default branch

[blairham]

Summary

• Run the advisory workflow on push to the repo default branch (in addition to PRs) and use pre-commit install-hooks to populate ~/.cache/pre-commit without executing any hook (so we don't replay org-wide debt on main).
• Gate the PR-only steps (PR-diff run, sticky comment, status reflection) behind github.event_name == 'pull_request'.

Why

GitHub Actions caches are scoped per branch — a PR branch can only restore caches from itself or the repo default branch. The previous job-level if: github.event_name == 'pull_request' meant the default-branch scope was never seeded, so every new PR branch rebuilt every hook env from scratch on its first push (the symptom reported: "pre-commit seems to be starting from scratch every time").

After this change, one merge to main seeds the cache, and subsequent PR branches restore via the exact key (when .pre-commit-config.yaml is unchanged) or the restore-keys prefix (when only some hooks change — partial rebuild). Same fix benefits the ~/.dotnet/tools cache (csharpier) and the pnpm store.

Caller-side follow-up

Callers that currently gate their pre-commit: job with if: github.event_name == 'pull_request' need to drop that guard so the reusable workflow triggers on push to main. Companion PR opening in pinpredict/trading as a reference.

Test plan

• Merge → confirm Pre-commit (advisory) runs on the merge commit in a consumer repo (e.g. trading) and the run log shows Install pre-commit hook envs (cache seed) step
• Open a new PR branch in the consumer repo → confirm the Cache pre-commit hook envs step reports a cache hit (or partial restore via restore-keys) and that pre-commit run skips env install
• Confirm no sticky PR comment is posted on push runs
• Confirm the advisory still posts/updates its sticky comment on PR runs

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