fix(tui): avoid placeholder session route on continue

[YOMXXX]

Issue for this PR

Closes #29262

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The TUI used sessionID: "dummy" as the initial route whenever --continue was passed. While the real previous session or fork target is resolved asynchronously, the session route can issue startup requests for /session/dummy, which fails server-side validation and shows an Unexpected server error toast.

This removes the placeholder session route. --continue and --continue --fork now start on the home route until the real session id is known. Explicit --session without --fork still starts directly on that real session id.

How did you verify your code works?

• bun test test/cli/tui/app.test.ts from packages/opencode
• bun test test/cli/tui/thread.test.ts from packages/opencode
• bun typecheck from packages/opencode
• bun turbo typecheck from repo root
• .husky/pre-push from repo root

Note: after syncing latest dev, the new stats packages required refreshing local dependencies with bun install so @tsconfig/node22 was available. The generated lockfile rewrite was not included in this PR.

Screenshots / recordings

Not applicable.

Checklist

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