Stop shipping megabytes of skill prompts in every snapshot refresh

The whole app turned to molasses because snapshot.load() shipped the
entire skill_runtime_snapshots table - unbounded, compiled_prompt
included - over IPC on every refresh. A real profile carried 142 rows
totaling 144MB (1.1MB each from the pre-cap CEO compile-all era), so
every coalesced refresh serialized ~144MB of JSON. The forced-humanizer
rework made it worse by adding a row per run for every agent.

- Drop skillRuntimeSnapshots, skillRevisions, skillTests, and
  skillBindings from the renderer snapshot: no renderer surface reads
  them; skill internals are main-process state. Only the lightweight
  skillDefinitions catalogue still ships.
- Project provider_requests without raw_response_json (4.1MB across the
  newest 100 rows; RunsPage only reads responseText)
- Migration 051 purges the bug-era compiled prompts from existing DBs
- resolveRuntimeSnapshot is now idempotent per run: a run re-queued by
  startup recovery reuses its frozen snapshot instead of dying on the
  uniqueness guard and burning real retry budget
- Regression test pins the contract: a 2MB database row must not add
  bytes to the snapshot (payload asserted under 400KB)

src/main/database/schema.ts

@@ -49,6 +49,7 @@ import { authConfigPurgeSchema } from "./migrations/047-auth-config-purge";
 import { uiPreferencesSchema } from "./migrations/048-ui-preferences";
 import { avatarsSchema } from "./migrations/049-avatars";
 import { financeSalesDepartmentsSchema } from "./migrations/050-finance-sales-departments";
+import { purgeRuntimeSnapshotBloatSchema } from "./migrations/051-purge-runtime-snapshot-bloat";
 
 interface Migration {
   version: number;
@@ -107,6 +108,7 @@ const MIGRATIONS: Migration[] = [
   { version: 48, name: "ui_preferences", sql: uiPreferencesSchema },
   { version: 49, name: "avatars", sql: avatarsSchema },
   { version: 50, name: "finance_sales_departments", sql: financeSalesDepartmentsSchema },
+  { version: 51, name: "purge_runtime_snapshot_bloat", sql: purgeRuntimeSnapshotBloatSchema },
 ];
 
 const SCHEMA_VERSION = MIGRATIONS[MIGRATIONS.length - 1].version;