fix: surface thinking and stabilize plugin proxy behavior

[hawkff]

Summary

• Ensure plugin-path thinking output is surfaced as reasoning_content:
  • SSE deltas use delta.reasoning_content
  • non-stream responses include message.reasoning_content
• Add --stream-partial-output to plugin cursor-agent calls so thinking deltas are emitted reliably.
• Improve plugin runtime behavior in OpenCode:
  • avoid duplicate assistant text chunks from non-partial snapshots
  • default to --force for headless tool execution (CURSOR_ACP_FORCE=false to disable)
  • map shell alias to bash in exposed tool list
  • stop forwarding raw tool_call chunks by default (prevents OpenCode type-validation failures); can be re-enabled with CURSOR_ACP_FORWARD_TOOL_CALLS=true

Why

OpenCode uses the plugin proxy path (not standalone). In that path, thinking output was not consistently visible and tool interactions could fail or behave inconsistently.

Validation

• Confirmed --thinking output now shows reasoning with cursor-acp/opus-4.6-thinking.
• Confirmed file create/delete prompts complete successfully through plugin path.

[hawkff]

• Before: opus-4.6-thinking in OpenCode did not show a visible Thinking block and tool actions were inconsistent.
• After this PR: Thinking appears in UI and the create+delete file flow completes successfully in the plugin path.

[Nomadcxx]

This is great, thank you. I've been actively working on tool mapping/action consistency. This is very complex.