fix(server): resolve the configured shell for session terminals - #51052

Issue for this PR

Fixes #51051 (#51051)

Type of change

• Bug fix

What does this PR do?

shell is documented as the shell used for the interactive terminal, and both the shell tool
(shell.ts:263) and pty.ts:166 resolve it through ShellSelect.Service. Session terminals did not:
persistent-pty/index.ts:160 falls back to the free ShellSelect.environment(), which reads only
process.env.SHELL and passes undefined in the slot where the configured options belong.
process.env.SHELL is normally unset on Windows and the Windows branch of select() puts
pwsh/powershell first, so a Windows session terminal started PowerShell whatever shell said, while the
agent's own commands ran in the configured shell.

persistentPty.create now resolves the configured shell and passes it down as command. It is resolved
in the handler rather than inside PersistentPty deliberately: PersistentPty is a process-global node
and ShellSelect.Service is Location-scoped, so the service cannot be yielded there —
handlers/config.ts:21 is the existing precedent for reaching it from a handler. Tell me if you would
rather have it as a resolver option on the PersistentPty layer and I will move it.

An explicit command in the request payload still wins, and non-HTTP callers keep the old fallback.

How did you verify your code works?

I did not run the repo checks: no Bun on this machine, so bun typecheck and bun run check never ran,
and no session terminal was actually opened. What I traced, at v2 tip 318a8c1:

• persistent-pty/index.ts:160 is the only consumer of ShellSelect.environment(), and the TUI's New Terminal action (tui/src/context/session-terminals.tsx:78-86) sends no command, so the fallback is the live path.
• handlers.ts builds ConfigHandler and PersistentPtyHandler in one list, and routes.ts:130 replaces ShellSelect.node with the configured variant, so the service a handler yields is the configured one.
• The only other create caller is the legacy handlers/pty.ts:47, which already resolves config via pty.ts:166.
• packages/server already imports @opencode/core/shell/select (handlers/config.ts:2), so no new dependency direction.

To confirm for real: set "shell" to a Git Bash path on Windows, open a session terminal, and see that
$PSVersionTable no longer answers.

Screenshots / recordings

No UI change.

Checklist

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

"Tested locally" covers the source trace above and nothing more — the handler itself was not executed.