feat(desktop): open workspace Markdown references

[1625567290]

Summary

• Make workspace-relative Markdown references clickable in assistant output and sent workspace-file chips, including encoded spaces and non-ASCII paths.
• Preview .md, .markdown, and .mdx files in the existing Files workbar while keeping the transcript mounted, with Open locally and Show in folder actions.
• Resolve paths in the main process against the session workspace realpath, reject traversal, schemes, symlink escapes, and non-Markdown files, and cap previews at 256 KiB.

Fixes #2664

Verification

• npm run typecheck
• npm run format:check
• npx biome lint --changed --since=origin/main
• npm run check:asf-headers
• npm --workspace @maka/ui test (236 passed)
• node --test apps/desktop/dist/main/tests/runtime-host-workspace-ipc-main.test.js apps/desktop/dist/main/tests/workspace-file-guard.test.js (3 passed)
• npm --workspace @maka/desktop run e2e -- e2e/workspace-markdown.spec.ts (1 passed)

The full local Desktop run reached 1504 passing tests. Eight existing MCP OAuth deadline tests were cancelled under local Node 22.22.1; the affected workspace tests and exact-head Electron E2E pass, and the OAuth source and test files are unchanged from main.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No