feat: layer runtime and model selection across conversation, project and global

- Store runtime selection as { provider?, model? } layers and resolve them per
  field (conversation -> project -> global); drop the legacy auto provider.
- Migrate stored selections (database v49) and clear deleted profile ids.
- Re-resolve selections in Main per request and at queue dispatch.
- Add ProjectRuntimeSelector and a two-column RuntimeModelPicker; manage
  link opens model connections; restore per-runtime default model tabs.
- Stop duplicating trusted instructions and history across OpenCode,
  Continue and DeepSeek Harness runtimes.
- Document the layered design, global provider removal plan and planned
  purpose-specific models.

src/shared/remote-project-candidate-contracts.ts

@@ -4,7 +4,7 @@ import {
   REMOTE_WORKSPACE_LIMITS,
   remoteAbsolutePathSchema
 } from './remote-agent-contracts'
-import { agentRuntimeSelectionSchema } from './runtime-selection-contracts'
+import { runtimeSelectionLayerSchema } from './runtime-selection-contracts'
 import { sshHostIdSchema } from './ssh-host-contracts'
 
 export const REMOTE_PROJECT_SAVE_LIMITS = {
@@ -30,7 +30,7 @@ const commonDraftFields = {
   name: projectCreateSchema.shape.name,
   description: projectCreateSchema.shape.description,
   defaultWorkMode: projectCreateSchema.shape.defaultWorkMode,
-  runtimeSelection: agentRuntimeSelectionSchema,
+  runtimeSelection: runtimeSelectionLayerSchema.optional(),
   hostId: sshHostIdSchema,
   remoteRootPath: remoteProjectRootPathSchema
 } as const