feat: add workspace agents for reusable agent definitions across patterns

Introduce a workspace-level agent library that allows users to define
agents once and reference them from multiple orchestration patterns.

Domain model:
- Add WorkspaceAgentDefinition type with name, model, instructions, etc.
- Extend PatternAgentDefinition with optional workspaceAgentId and overrides
- Add resolution helpers that merge workspace agent base with per-pattern overrides
- Extend WorkspaceSettings with agents array and normalize on load

IPC & main process:
- Add saveWorkspaceAgent/deleteWorkspaceAgent IPC channels and handlers
- Resolve workspace agent references in buildEffectivePattern before
  sending to sidecar (no C# changes needed)

Settings UI:
- Add 'Agents' tab under Orchestration in the Settings panel
- Create WorkspaceAgentEditor component using ToolingEditorShell
- Show usage count (which patterns reference each agent)

Pattern editor integration:
- Add agent picker dropdown: 'New inline agent' or 'From library'
- Show linked badge (chain icon) on referenced agent graph nodes
- Show linked workspace agent banner in the inspector
- Add 'Save to Agent Library' action to promote inline agents
- Add 'Unlink' action to convert referenced agents back to inline

Tests:
- Add unit tests for resolution helpers (resolvePatternAgent,
  resolvePatternAgents, findWorkspaceAgentUsages, normalize)
- Update existing tooling test for new agents field

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/preload/index.ts

@@ -37,6 +37,8 @@ const api: ElectronApi = {
   deleteMcpServer: (serverId) => ipcRenderer.invoke(ipcChannels.deleteMcpServer, serverId),
   saveLspProfile: (input) => ipcRenderer.invoke(ipcChannels.saveLspProfile, input),
   deleteLspProfile: (profileId) => ipcRenderer.invoke(ipcChannels.deleteLspProfile, profileId),
+  saveWorkspaceAgent: (input) => ipcRenderer.invoke(ipcChannels.saveWorkspaceAgent, input),
+  deleteWorkspaceAgent: (agentId) => ipcRenderer.invoke(ipcChannels.deleteWorkspaceAgent, agentId),
   describeTerminal: () => ipcRenderer.invoke(ipcChannels.describeTerminal),
   createTerminal: () => ipcRenderer.invoke(ipcChannels.createTerminal),
   restartTerminal: () => ipcRenderer.invoke(ipcChannels.restartTerminal),