fix: prevent ambiguous slash phrases from rendering as file links

[alphali]

Synopsis

CodexMonitor was incorrectly turning natural-language slash phrases like app/daemon,
Git/Plan, and Spec/Verification/Evidence into file links in rendered messages.

The raw agentMessage.text from thread/resume response was correct, so the issue was in
frontend markdown rendering, specifically the plain-text file linkification logic in
remarkFileLinks.

Before

• natural-language slash phrases were rendered as file links

After

Changes

• make plain-text file linkification more conservative
• stop treating ambiguous slash phrases as file paths
• keep explicit file-like paths such as docs/setup.md linkable
• add regression tests for markdown rendering behavior

Validation

• npm run test -- src/features/messages/components/Markdown.test.tsx
• npm run typecheck
• manual verification in npm run tauri:dev

[Samigos]

Since you are on it, can you try fixing the bug where if a project name contains a space it doesn't render correctly on the message?