fix: preview absolute Markdown links in sidebar

[whtml55]

Summary

• make complete absolute .md / .markdown inline-code paths clickable while preserving the stock produced-file resolver first
• route clicks through remote.session.openWorkspacePath into the sidebar
• authorize external Markdown with a random, session-bound, exact-canonical-path read capability minted only for loopback TCP peers
• force external preview tabs read-only at both the viewer UI and CodeMirror transaction layers while leaving writes and all unrelated paths workspace-fenced

Validation

• pnpm typecheck
• pnpm lint
• pnpm test (121 files, 1291 passed, 9 skipped)
• pnpm build
• pnpm check:consumer-types
• pnpm peers check