Harden GitHub Actions against supply-chain attacks

[alii]

Hardens our CI against the supply-chain attack patterns going around (Shai-Hulud worm, the tj-actions/changed-files token theft, and the pull_request_target + malicious .claude/settings.json hook that hit aidenybai/million and TanStack).

Audit results

• ✅ No pull_request_target anywhere (the vector that hit Million/TanStack)
• ✅ No malicious .claude / editor hook; .vscode/settings.json only sets the Prettier formatter
• ✅ No committed .npmrc / npm token — we publish via OIDC provenance (id-token: write + NPM_CONFIG_PROVENANCE), so the "npm invalidated all tokens" event doesn't affect us

Changes

1. Pin every action to a full commit SHA (with a version comment so Dependabot still bumps them). Tags like @v6 are mutable and can be repointed at malicious code — changesets/action@v1 was actually a branch ref, running in our publish job with id-token: write.
2. Harden bun install — --frozen-lockfile everywhere (install exactly the reviewed lockfile), plus --ignore-scripts in the privileged release job so dependency lifecycle scripts (the Shai-Hulud propagation vector) can't run next to publish credentials.
3. Gate the /snapshot trigger in snapshot.yml on author_association (OWNER/MEMBER/COLLABORATOR) + startsWith, so it's no longer triggerable by arbitrary commenters.
4. Trim permissions — drop unused issues: write from publish.yml; replace the dead top-level pull-requests: write in linters.yml with contents: read.
5. Remove the third-party LouisBrunner/checks-action in favor of a native $GITHUB_STEP_SUMMARY write — no third-party dep, no token, and drops checks: write from the lint job.

🤖 Generated with Claude Code