feat(web): open and edit linked workspace files

[GTC2333]

Summary

Stage 3 PR 9/9.

• recognize managed workspace/data file references in assistant Markdown
• open linked files directly in the file sidebar
• support inline editing with dirty-state and conflict protection
• add delete confirmation and safer file navigation
• keep file-sidebar state scoped to the active session
• improve responsive file-sidebar behavior

Verification

• root typecheck
• 24 targeted Web tests
• Web production build
• diff whitespace validation

Merge notes

Targets main directly. If PR 7 merges first, this branch may need a small rebase because both touch the composer and shell UI.