ci(t3x): add fork-runnable PR/main verify gate

[radroid]

Why

Upstream's ci.yml runs on blacksmith-* runners the fork can't access, so fork PRs currently get no checks — the "no independent CI gate" gap flagged in the review of the last sync PR (#22). Until now, "green" on a sync was only self-attested by the resolve job.

What

New fork-owned workflow .github/workflows/t3x-ci.yml — vp run typecheck / lint / test on ubuntu-latest:

• pull_request → gates every feature PR (like this one).
• push to main → post-sync verification that landed main is green.
• push to t3x/sync-** → independent gate for sync branches (a conflicting sync PR can't build a merge ref, so pull_request never fires on it).

All three checks run each time (if: ${{ !cancelled() }}) so one run reports every failure. Minimal contents: read permissions, no secrets. Also listed t3x-sync-resolve.yml + t3x-ci.yml in SEAMS.md's fork-owned files.

Self-test

This PR adds the workflow, so the t3x fork CI check on this very PR is the proof it works — merging once it's green.

🤖 Generated with Claude Code

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch t3x/fork-ci-pr-gate

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