fix: keep sessions discoverable for non-git projects on Windows

[BB-84C]

Issue for this PR

Closes #35750

Type of change

• Bug fix

What does this PR do?

Sessions created in non-git projects on Windows can disappear from the session picker.

Non-git projects use worktree "/" (set in Project.fromDirectory). On Windows, path.resolve("/") resolves to the drive root of the current process, so sessionPath() produced a machine-dependent absolute path like D:/repo and stored it in session.path. The session list query matches paths relative to the worktree, so those rows never matched and the sessions vanished from the picker. Which sessions were visible depended on which drive root the server process happened to be rooted at. Related reports: #37041, #37353, #38780.

Changes:

• sessionPath() returns undefined when the worktree-relative result is absolute (cross-volume, which is what happens for non-git worktrees on Windows). Sessions without a usable path fall back to the existing directory-based scoping in listByProject (path IS NULL AND directory = ?).
• The TUI session list query now passes directory alongside path, which is what makes the directory fallback reachable for project instances. POSIX behavior and git-project behavior are unchanged (the path condition still matches first).
• Migration 20260804120000_normalize_session_path NULLs out legacy absolute/empty paths so existing sessions become visible again after upgrade.

How did you verify your code works?

• New unit tests for sessionPath (win32 cross-volume returns undefined, same-volume keeps relative, posix unchanged) in packages/opencode/test/session/session-path.test.ts.
• New server test in packages/opencode/test/server/session-list.test.ts: a NULL-path session is listed when directory is provided and stays hidden without it. 12/12 pass in that file.
• tsgo --noEmit passes for opencode, tui, and core; oxlint clean on changed files.
• Real-machine check on Windows (non-git directory D:\BB84.ai): built the fork binary, pointed it at a snapshot copy of a real production DB (16,873 sessions, including rows with stored absolute paths like D:/BB84.ai and empty paths), enabled session directory filtering in the TUI, and confirmed sessions remain visible instead of being lost. The migration was exercised on the snapshot only; the production DB was not modified.

Screenshots / recordings

No screenshots - verification was done interactively in the TUI.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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