ci: auto-release on package.json version bump

[jonathan-pap]

Summary

Closes the gap between `package.json` version bumps and the GitHub Releases page.

Tagging stalled after v0.8.1 — PRs kept updating `package.json` + adding `changelog/<x.y.z>.md` shards, but no releases were ever filed. v0.9.0 through v0.11.7 were just backfilled by hand (11 tags + 11 releases). This workflow makes sure the next release happens the same day it merges.

How it works

On every push to `main` that touches `package.json` or `changelog/*.md`:

1. Diffs `package.json` version against the previous commit; skips if unchanged.
2. Skips if `v<x.y.z>` tag already exists (idempotent on re-runs).
3. Refuses if `changelog/<x.y.z>.md` doesn't exist (no silent notes-less releases).
4. Tags `v<x.y.z>` annotated with the theme line from the changelog file.
5. Creates a GitHub Release: title `v<x.y.z> — `, body = `changelog/<x.y.z>.md`.

Test plan

• First merge to main with a version bump should fire the workflow and create the release. The next time you bump (e.g. when the scheduled v0.12.0 agent merges its PR on May 4), the GitHub Releases page should auto-populate with v0.12.0 — no manual step.
• Path filter limits trigger surface to `package.json` + `changelog/*.md` so it doesn't run on every commit.
• Workflow uses `fetch-depth: 0` so the prev-version diff has access to history.

Permissions

Adds `permissions: contents: write` (default is `read`) — required for the workflow to push tags and create releases via `GITHUB_TOKEN`.

🤖 Generated with Claude Code