fix(tui): label Windows folder session tabs

[opencode-agent]

Summary

• recognize Windows drive roots when resolving project labels
• fall back to each session directory basename for non-repository tabs
• add Windows folder fixtures to the session-tabs story and a cross-platform regression test

Checks

• bun test test/util/project.test.ts test/context/session-tabs-model.test.ts
• bun typecheck
• bun x prettier --check packages/tui/src/util/project.ts packages/tui/test/util/project.test.ts packages/tui/src/feature-plugins/system/storybook/session-tabs.tsx
• OpenCode Drive before/after capture at 100×35 cells

Requested by: @kitlangton (Kit via Slack)

[Enough1122]

AI code review — automated review for reference, author can ignore or act on any point.

Right root cause and a clean fix: path.win32.basename("C:\\") returns an empty string, which is why drive-root projects lost their tab labels, and choosing the parser family up front via /^(?:[A-Za-z]:[\\/]|\\\\)/ correctly covers drive letters (mixed separators included), UNC shares, and keeps POSIX names containing backslashes intact — the new regression test documenting that last nuance is a nice touch.

Two suggestions:

1. Explicit name is still ignored for root projects (packages/tui/src/util/project.ts, the if (paths.parse(canonical).root === canonical) branch): it returns basename(fallback) before ever consulting project?.name. A workspace that has an explicit name but whose canonical path is a filesystem root gets relabeled by whichever directory one session happened to open. If that ordering wasn't deliberate, consider return project?.name ?? (fallback ? paths.basename(fallback) : undefined) for the root branch. At minimum, a test pinning the intended precedence would prevent future churn.

2. Test coverage gaps worth locking down: (a) drive root with no fallback → expected undefined; (b) a lowercase drive letter (c:\) exercising the i flag; (c) UNC canonical (\\server\share) — path.win32.parse treats the whole share as .root, so it takes the fallback path; asserting that behavior makes the UNC handling intentional rather than accidental.

Minor: the storybook fixture list now mixes two shapes and relies on TS inferring a union plus the "project" in tab narrowing — works fine today; an explicit fixture type would keep it stable as fixtures grow.