Splits the three grouped router files into eight api_<verb>_<resource>.py
modules under decnet/web/router/swarm/ to match the convention used by
router/fleet/ and router/config/. Shared request/response models live in
_schemas.py. Keeps each endpoint easy to locate and modify without
stepping on siblings.
Adds decnet/web/swarm_api.py as an independent FastAPI app with routers
for host enrollment, deployment dispatch (sharding DecnetConfig across
enrolled workers via AgentClient), and active health probing. Runs as
its own uvicorn subprocess via 'decnet swarmctl', mirroring the isolation
pattern used by 'decnet api'. Also wires up 'decnet agent' CLI entry for
the worker side.
29 tests added under tests/swarm/test_swarm_api.py cover enrollment
(including bundle generation + duplicate rejection), host CRUD, sharding
correctness, non-swarm-mode rejection, teardown, and health probes with
a stubbed AgentClient.
