[FEATURE]: /tool, /status, /exec server endpoints

[micuintus]

• I've checked that this feature hasn't been suggested before.

TL;DR

Add three new server API endpoints: /tool (inject external tool results), /status (report status/progress), and /exec (execute tools by name). These enable plugins to interact with sessions programmatically.

More detail

• POST /session/:id/tool — Injects an external tool call + result into a session. Uses the external flag on ToolPart to mark results as externally produced. Depends on the external flag PR.
• POST /session/:id/status — Appends a status/progress message to a session. Useful for plugins to report progress of long-running operations.
• POST /session/:id/exec — Executes a registered tool by name with given arguments, returning the result. Enables plugins to invoke opencode's built-in tools programmatically.

Part of the plugin primitives work tracked in #20018 (split from #21687).

[github-actions]

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What needs to be fixed:

• Feature request titles must start with [FEATURE]: (e.g., [FEATURE]: /tool, /status, /exec endpoints)

Please edit this issue to address the above within 2 hours, or it will be automatically closed.

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[github-actions]

Thanks for updating the issue! Everything looks good now — the title correctly starts with [FEATURE]:, the checkbox is checked, and the description clearly explains the proposed endpoints and their purpose. The issue is now compliant.