fix(opencode): handle empty session list for attach --dir to prevent segfault

[lexlian]

Issue for this PR

Closes #28214

Type of change

• Bug fix

What does this PR do?

opencode attach <url> --dir <PATH> crashes with a Bun segmentation fault when no existing session on the server has a working directory matching <PATH>.

Root cause: validateSession() in validate-session.ts only validates when --session is provided. When --dir is provided without --session, validation is skipped entirely. The TUI then starts with an empty session list filtered by the directory, causing an unhandled rejection that triggers Bun's microtask queue to panic.

Fix (two parts):

1. validate-session.ts: Added directory validation — when --dir is provided without --session, fetch the session list and throw a clear error if empty.
2. app.tsx: Added graceful handling in the --continue effect — when no sessions are found for the directory, navigate to home instead of staying on the dummy session route.

How did you verify your code works?

• Created 5 unit tests in test/cli/validate-session.test.ts — all pass
• Both modified files compile cleanly with bun build
• The fix catches the error early with a clear message instead of segfaulting

Screenshots / recordings

N/A — CLI fix, not a UI change.

Checklist

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

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