fix: clean up orphaned processes and LSP state on shutdown

[nianyi778]

Issue for this PR

Closes #12767
Related to #18632

Type of change

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

What does this PR do?

This fixes two shutdown paths that still look leaky on current dev.

First, packages/opencode/src/index.ts does not register process-level handlers for SIGTERM, SIGHUP, or SIGPIPE. The TUI has its own SIGHUP handling, but commands like opencode run, opencode acp, and opencode serve do not. This PR adds a small signal.ts helper that calls Instance.disposeAll() and then exits, with a 5 second timeout in case cleanup hangs.

Second, LSP shutdown was stopping the child process but keeping in-memory state alive. This clears the LSP diagnostics map, file version map, and the top-level LSP client caches during shutdown.

I also clear the prompt msgs array right after processor.process(...) returns, since that array can get large and is not needed for the rest of the loop iteration.

I did not copy over older PR changes around plugin subscriptions or processor cleanup because current dev already uses scoped Effect subscriptions there, and SessionProcessor already runs cleanup through Effect.ensuring(cleanup()).

How did you verify your code works?

I tested the signal path locally with the ACP command, since it is a long-running non-TUI entrypoint:

• started bun run ./src/index.ts acp
• sent SIGHUP
• confirmed the process exited
• started it again
• sent SIGTERM
• confirmed the process exited

I also tried to run bun typecheck in packages/opencode, but this checkout currently does not have tsgo installed in node_modules/.bin, so the script exits before typechecking starts.

Screenshots / recordings

N/A

Checklist

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

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