ci: add cross-platform test matrix (macos + windows + linux)

[bezata]

Summary

• New `.github/workflows/ci.yml` runs `typecheck + lint + test + build` across `ubuntu-latest`, `macos-latest`, and `windows-latest` on every PR and every push to main.
• Lands standalone on the v0.2.5 baseline (no feature changes here) so we establish a confirmed green cross-platform baseline before v0.3.0's vault-discovery work, which will add real platform-sensitive path resolution code.
• Existing `release.yml` is untouched — it still runs its own test gate on `ubuntu-latest` inside `prepublishOnly` at tag-push time. This workflow sits on top, keeping main green between releases.

Why

Today we have no cross-OS CI at all. A Windows-only or macOS-only regression could sit in main for weeks unnoticed. The upcoming `vault.*` feature (v0.3.0) introduces `os.homedir()` + `process.platform` branching for Obsidian's `obsidian.json` discovery — exactly the kind of code that silently breaks on one OS if you only test on another.

Cost

• ~3 min wall-clock per PR (three runners in parallel, `fail-fast: false`).
• `cancel-in-progress: true` on concurrency means superseded runs are killed when you push new commits, keeping queue pressure low.

Test plan

• YAML syntax validated locally
• Confirm workflow triggers on this PR and all three matrix legs go green
• After merge, confirm green on `push: main`

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