feat: Phase 3 executor with SSE streaming

[prosdev]

Summary

• Add graph executor with SSE streaming, run lifecycle management, and human input pause/resume
• Add run API routes: POST /graphs/{id}/run, GET /runs/{id}/stream, POST /runs/{id}/resume, GET /runs/{id}/status
• Add checkpointer parameter to build_graph for interrupt/resume support
• Split code-reviewer agent into 3 specialized parallel agents (security, logic, quality)
• Move manual test scripts from scripts/ to tests/manual/

Test plan

All 18 manual tests pass (bash tests/manual/run_all.sh):

Phase 2 builder (existing, moved to tests/manual/)

• test_01 — Linear graph (FakeListChatModel)
• test_02 — Real Gemini LLM integration
• test_03 — Branching with field_equals condition
• test_04 — Tool node + tool_error condition routing
• test_05 — Human input interrupt & resume
• test_06 — Full pipeline (tool + LLM + condition)

Phase 3 executor + SSE (new)

• test_07 — SSE event sequence: run_started → node_started → node_completed → edge_traversed → graph_completed
• test_08 — State snapshots in node_completed events evolve across nodes
• test_09 — Run status transitions (running → completed), duration_ms, final_state
• test_10 — Human input pause/resume via executor: graph_paused event, submit_resume, completion
• test_11 — SSE reconnection: replay buffer stores sequential IDs, Last-Event-ID skips seen events
• test_12 — Concurrent run limit: MAX_RUNS_PER_KEY enforced, different owners independent
• test_13 — Run timeout: RUN_TIMEOUT_SECONDS triggers error event, loop terminates
• test_14 — Condition routing SSE: edge_traversed shows correct condition_result per branch
• test_15 — Tool error routing SSE: tool_error routes success→END, error→llm_err with deferred edge
• test_16 — Keepalive during pause: no id field, excluded from replay buffer
• test_17 — DB fallback: format_sse produces correct terminal events for completed/lost runs
• test_18 — Cancel run: cancel_event terminates execution, error event emitted

Notable findings during testing

• Deferred condition edges don't emit edge_traversed when routing to END (not a real node) — test_15 verifies via completed node list instead
• Cancel during pause requires both cancel_event + resume_event to unblock _wait_for_resume — test_18 documents this
• SSE stream_run_sse queue is single-reader; reconnection test_11 validates replay buffer directly

🤖 Generated with Claude Code