feat: standardize HTTP response shapes on snake_case

[ethanj]

Summary

• Establish snake_case as the canonical wire format for all HTTP responses, matching the request-side convention already in place and aligning with the AI API ecosystem (Anthropic, OpenAI, Stripe) that target OSS adopters are already integrated with.
• Ingest response splits memory IDs by outcome (stored_memory_ids / updated_memory_ids). The accumulator previously lumped all touched IDs into one array, so callers couldn't distinguish creates from updates — now each list length matches its corresponding count.
• New src/routes/memory-response-formatters.ts centralizes the translation layer; internal TS types stay camelCase, formatters are the only seam that touches the wire. All leaky routes that were passing internal types directly to res.json() now go through explicit formatters.
• Search response's nested observability (retrieval / packaging / assembly traces) and scope echo flipped to snake_case — previously leaked camelCase via passthrough of internal summaries.

What changed

• Ingest: emit episode_id, facts_extracted, memories_stored, memories_updated, memories_deleted, memories_skipped, stored_memory_ids, updated_memory_ids, links_created, composites_created.
• Search: scope echo → { kind, user_id, workspace_id, agent_id, agent_scope }. Observability nested fields snake_cased (package_type, included_ids, dropped_ids, evidence_roles, has_current_marker, final_token_cost, primary_evidence_position, etc.).
• Other routes: lesson_id, memory_id / version_count on audit trail, snake_case applied array on PUT /config, and explicit formatters for /stats, /consolidate, /decay, /cap, /reset-source, /reconcile, /lessons/stats, /audit/summary, /audit/recent.
• Internal types: IngestResult gains storedMemoryIds + updatedMemoryIds; keeps memoryIds (union) for post-write processors that iterate over every touched memory regardless of outcome.

Follow-ups (out of scope)

• OpenAPI spec declares only request bodies today. Adding response schemas would let npm run check:openapi catch future wire drift automatically.

Test plan

• npx tsc --noEmit clean
• All 28 HTTP-seam tests pass end-to-end (route-validation, research-consumption-seams, composed-boot-parity, versioned-mount, smoke)
• npm run generate:openapi produces zero diff against checked-in spec
• SDK companion PR (atomicmemory-sdk snake-case-wire-convention) merged in lockstep — maps the new snake_case shapes back to camelCase for SDK consumers