docs: polish OSS project surfaces

[harshmathurx]

Summary

Refreshes OpenNotes' public OSS surfaces so new users and contributors can understand the project faster: status, install caveats, data model, contribution paths, architecture, support, governance, and release process.

Why

The repo already had the right OSS basics, but the entry points were uneven. The README mixed product pitch, download notes, and contributor guidance without a clear maintainer map, and the changelog/release docs had drifted from the current release state.

Changes

• Reworked the README with a clearer status section, download/install caveat, data-location summary, extension entry points, project map, and roadmap.
• Added docs/architecture.md for contributors changing the app shell, core services, extensions, or Tauri bridge.
• Added SUPPORT.md and GOVERNANCE.md.
• Expanded CONTRIBUTING.md with project orientation and extension contribution guidance.
• Tightened SECURITY.md around local-first risks, bridge scope, and private disclosure.
• Updated CHANGELOG.md for v0.1.1, v0.1.2, and the current unreleased docs pass.
• Fixed stale release instructions around version numbers and prerelease handling.
• Removed local machine paths from an old planning doc.
• Added issue-template contact links for security and support.

Validation

• pnpm exec prettier --check README.md CONTRIBUTING.md SECURITY.md RELEASE.md CHANGELOG.md docs/README.md docs/architecture.md docs/plan.md GOVERNANCE.md SUPPORT.md .github/ISSUE_TEMPLATE/config.yml
• Markdown local-link check script
• git diff --check
• pnpm run lint
• pnpm run typecheck
• pnpm exec vitest run (24 files, 386 tests)
• pnpm run build

Release note

N/A. Documentation-only OSS polish.