Adds the Phase 3 UI for the workspace file viewer:
- FileViewerProvider/FileViewerSheet pair, URL-addressable via
?file=/?line=/?column=/?workspace= so browser back closes or
walks back through sheet history.
- Remark plugin + parser that only auto-link path refs inside
inline code spans (requires a slash or a line anchor to avoid
prose matches like README.md).
- Shared WorkspaceFileLink primitive used by markdown output; same
primitive will be reusable for run output and work-product chips.
- Issue toolbar "Open file" action, Command Palette entry
"Open file in this issue...", and the `g f` keyboard route.
- Designed state views for not found / outside workspace / denied /
too large / binary / remote-only / workspace-archived / loading
progressive states, with polite live-region announcements and a
keyboard-scrollable code viewport.
Tests cover the parser, the remark plugin, URL state reading/writing,
the keyboard shortcut, and markdown-body workspace link rendering.
Co-Authored-By: Paperclip <noreply@paperclip.ing>
