release: align desktop release versions

[fettpl]

Summary

• Adds a semantic-release prepare step that applies nextRelease.version to root and frontend package metadata in the release workspace.
• Adds a tested scripts/apply-release-version.mjs helper so future desktop release builds can use metadata aligned with the GitHub release version.
• Documents semantic-release as the stable desktop release version source of truth.

Context

Addresses #148 and supports the broader Homebrew-readiness umbrella in #140.

This keeps the workflow compatible with protected main: it does not require the release bot to commit package-version bumps back to the repository. Instead, release-time package metadata is updated in the GitHub Actions workspace before future desktop artifact packaging/publishing work.

Review Notes

• The branch was rebased onto current main after review so the PR now has a single relevant commit and no merge-commit noise.
• The @semantic-release/exec prepareCmd syntax was checked against the plugin documentation; command strings are template-rendered with semantic-release context values such as nextRelease.version.
• The PR intentionally does not publish desktop artifacts. That remains tracked by ci(release): publish signed macOS desktop assets #147.

Test Plan

• node --test scripts/test-apply-release-version.mjs failed first with ERR_MODULE_NOT_FOUND before the helper existed.
• npm run test:release-version
• npm run check:controller && npm run check:cli
• Pre-push frontend quality gate passed while force-pushing the rebased branch:
  • node scripts/validate-package-json.mjs
  • npm run check:static
  • npm run check:cleanup
  • npm run build

Known unrelated validation note:

• npm run check:contracts currently fails on rebased main because AggregatedSession is exported in both frontend/src/app/api/agent/sessions/all/route.ts and frontend/src/features/agent/session-contracts.ts. This PR does not touch either file.

AI Disclosure

AI assistance was used to inspect the release/versioning setup, draft the helper and tests, run validation commands, and perform the follow-up review. I reviewed the resulting diff and command output before opening and updating this PR.