Add swarm sessions: durable agent pools, scoped messaging, and Modal-backed workers

[ReganBell]

Implements swarm sessions per Leo's design doc: durable agent pools ("swarms") rooted in a session, scoped worker messaging, and Modal-backed private worker sandboxes. Explicitly excludes the filesystem snapshot/copy/restore portion of the design.

What's included

• src/swarms/ — swarm service, durable store, and lifecycle (reserve → provision → ready → deliver → cleanup) with bounded reconciliation sweeps, separate resource/delivery pools, and per-swarm advisory locking.
• API surface in src/api/routes/swarms.ts with action-specific field allowlists; agent operations require an active capability run (matching thread, actor, scope, unexpired lease).
• Orchestrator integration: approvals and security-screen grants are session-isolated — a root-session grant never authorizes a worker.
• Worker sandbox routing honors the recorded per-worker sandboxId (no silent fallback to the scope default).
• docs/swarms.md.

Validation

• 397 repo tests pass (swarms, swarm-api, swarm-modal, swarm-orchestrator, sandbox-resources, sessions-background-activity, orchestrator, sandbox-routing, worker-reaper, run-result-delivery, background-exec-broker, modal-sandbox, durable-stores, postgres-swarms), including new race regressions against real Postgres.
• Static checks pass: typecheck, typecheck:contract, lint, lint:ox, lint:knip; prettier and git diff --check clean.
• Four adversarial review rounds against the combined diff, each in a fresh context, with independent read-only probe batteries (memory + real Postgres). Rounds 1–3 produced nine findings (capability authority, approval isolation, roster gating, sandbox routing, sweep bounds/starvation, identity-field rejection, pending-query single-flight, docs, and a reconciliation-timeout rollback race); all nine are fixed and re-verified at this HEAD. Round 4 adjudicated all nine FIXED with no new major findings and approved.
• Modal provisioning is mocked in tests; no real provisioning was performed.

QA status

Slack/dev-instance QA is blocked: no dev Slack pool credentials, no Modal tokens, and dev-instance boot failed on missing model creds. Live end-to-end QA on a dev instance should happen before merge.

Draft until live QA and human review.

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