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/contracts.ts

@@ -228,7 +228,7 @@ import type {
   SshHostValidationResult
 } from './ssh-host-contracts'
 import {
-  agentRuntimeSelectionSchema,
+  optionalAgentRuntimeSelectionSchema,
   type AgentRuntimeSelection
 } from './runtime-selection-contracts'
 import {
@@ -343,7 +343,7 @@ export const agentRequestSchema = z
     teamMode: z.boolean().optional(),
     smartRouting: z.boolean().optional(),
     queueItemId: z.string().uuid().optional(),
-    runtimeSelection: agentRuntimeSelectionSchema.optional(),
+    runtimeSelection: optionalAgentRuntimeSelectionSchema,
     runtimeControl: runtimeControlSchema.optional(),
     workMode: legacyWorkModeSchema.optional(),
     prompt: z.string().trim().min(1).max(100_000),
@@ -386,7 +386,8 @@ export const conversationQueueUserInputSchema = z
   .object({
     conversationId: conversationIdSchema,
     projectId: z.string().uuid().optional(),
-    runtimeSelection: agentRuntimeSelectionSchema,
+    /** Informational only; the renderer re-resolves the selection at dispatch. */
+    runtimeSelection: optionalAgentRuntimeSelectionSchema,
     runtimeControl: runtimeControlSchema.optional(),
     expertId: z.string().uuid().optional(),
     teamMode: z.boolean().optional(),