fix: terminate local stdio mcp processes on disconnect / replace / rollback

[yangshuai0711]

Issue for this PR

Closes #26714
Related to #15808

Type of change

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

What does this PR do?

Local stdio MCP servers were leaking processes on disconnect, replace, and rollback. PR #15516 added descendant cleanup in the dispose handler but left three gaps:

1. The root PID was never killed, only descendants
2. disconnect() only called client.close() with no kill at all
3. Rollback after defs() failure also skipped the kill

This pulls teardown into a single stopClient() helper that kills descendants, kills the root PID, then calls client.close(). It is reused across all four paths: disconnect, add-replace, defs-failure rollback, and instance disposal.

How did you verify your code works?

• bun test test/mcp/lifecycle.test.ts in packages/opencode — 26 pass, 0 fail
• Built and ran a release binary locally, no regressions

Checklist

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

[UncleXJ]

This PR addresses the exact MCP process leak I've been experiencing on macOS desktop. After a few days of use, I had 27 zombie MCP processes (minimax, agentmemory, zai-mcp) consuming ~500MB, all orphaned children of the OpenCode Helper Electron process.

The root cause matches this PR perfectly: disconnect/replace/defs-failure paths don't kill spawned processes, only client.close() is called.

Would love to see this merged - it's a significant memory leak for desktop users who keep OpenCode running for days. Happy to help test if needed.

Related: #26714

[UncleXJ]

+1, this PR fixes a real memory leak affecting desktop users. I've confirmed 27+ zombie MCP processes accumulating over days (minimax, agentmemory, zai-mcp) all orphaned under OpenCode Helper. The fix in this PR covering all 4 paths (disconnect, replace, defs-fallback, disposal) would eliminate this. Hope this can get reviewed and merged soon. @thdxr

[UncleXJ]

+1, confirmed 27 zombie MCP processes (minimax, agentmemory, zai-mcp) leaking under OpenCode Helper on macOS desktop. This PR covers all paths needed for cleanup. Please review and merge. @thdxr

[UncleXJ]

+1 — confirmed 27 zombie MCP processes (minimax, agentmemory, zai-mcp) leaking under OpenCode Helper on macOS desktop. Current workaround: periodic cleanup LaunchAgent + migrated minimax to Streamable HTTP. Root fix needed for all stdio MCP types.

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