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/services/skill-service.ts

@@ -178,9 +178,12 @@ export class SkillService {
    * compiled prompt instead of silently overflowing the model's context.
    */
   resolveRuntimeSnapshot(input: ResolveAgentSkillsInput): string {
+    // Idempotent per run: a run interrupted after resolving (crash, quit) is
+    // re-queued by startup recovery and must reuse its frozen skill set, not
+    // die on a uniqueness error that burns real retry budget.
     const existingSnapshot = this.db.prepare("select id from skill_runtime_snapshots where agent_id = ? and run_id = ?")
       .get(input.agentId, input.runId) as { id: string } | undefined;
-    if (existingSnapshot) throw new Error("Skill runtime snapshot already exists for this run.");
+    if (existingSnapshot) return existingSnapshot.id;
     const forcedRows = this.db.prepare(`
       select
         sd.id as skill_id,