fix(sessions): strip remote workspace prefix from file paths in chat

[timgl]

Summary

When viewing a cloud task, file chips in the chat rendered the full sandbox path (e.g. /tmp/workspace/repos/posthog/posthog.com/src/c). For cloud tasks the local workspace cwd is empty (folderPath: "", worktreePath: null), so useCwd() returned undefined and FileMentionChip's toRelativePath had nothing to strip.

This adds a small useDisplayRepoPath(taskId) hook that prefers the local cwd, and falls back to the conventional sandbox root /tmp/workspace/repos/<owner>/<repo> derived from task.repository when there isn't one. FileMentionChip uses it instead of useCwd so the chip shows just the repo-relative directory on cloud tasks.

• Read/Edit views are unchanged structurally — they already use FileMentionChip; the chip just gets a correct prefix to strip now.
• Local tasks keep the existing behavior (cwd takes precedence).
• The ToolCallView fallback (Glob/Grep/etc. via compactHomePath(title)) is unchanged — out of scope for this fix.

Test plan

• pnpm vitest run src/renderer/features/sessions — 161 tests pass, including 7 new tests for useDisplayRepoPath covering local cwd, derived remote path, missing/malformed repository, and missing task.
• pnpm exec tsc -p tsconfig.web.json --noEmit — clean.
• biome check — clean (auto-formatted on commit via lint-staged).
• Manually: open a cloud task that has performed a Read, verify the chip shows src/... rather than /tmp/workspace/repos/<owner>/<repo>/src/....
• Manually: open a local task and verify nothing regresses.

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Created with PostHog Code

[timgl]

I think we could use a comment here to explain what/why (brief!)