File-change diff cards from other sessions leak into a session's tool-return stream (no session isolation)

Describe the bug
When multiple opencode sessions share the same workspace, file-change summary cards (the collapsed "X file changed +N −M" diff cards) from other sessions are incorrectly appended to the end of this session's tool-return output.

To Reproduce

1. Open two opencode sessions on the same workspace (e.g. C:\...\代理软件\, which contains an untracked nethelper/ directory).
2. In session A, actively edit files under nethelper/ (e.g. C++ headers).
3. In session B (which never touches nethelper/), run any tool call.
4. Observe: after session B's tool returns, a collapsed card like nethelper/src/foo.h +2 −0 appears — showing session A's edits.

Expected behavior
Each session should only surface file-change cards for files it itself modified. Diff cards for another session's edits must not be rebroadcast into a different session's stream.

Root cause (hypothesis)
The file-watcher / session manager broadcasts workspace-wide change events to all sessions sharing a workspace, instead of scoping events per session. The UI is just the symptom; the leak is in the core event layer (session isolation), not the frontend.

Environment

• OS: Windows 11 (Hyper-V VM)
• opencode: desktop build (no TUI/CLI)
• Workspace includes a csharp LSP (OmniSharp) and plugins @plannotator/opencode, open-controller
• Note: a similar cross-session leak also occurs for LSP diagnostics (OmniSharp errors from session A's files show in session B).

Additional context
Both the per-session file-change cards and the cross-session LSP diagnostics point to the same missing isolation: workspace-scoped state / shared processes are not partitioned per session.