Move enterprise/aibridged and enterprise/aibridgedserver under coderd/
so the daemon can run on AGPL builds, and so chatd can route LLM traffic
through aibridge without depending on enterprise code.

The aibridged daemon has no enterprise-only dependencies (the only
runtime touchpoint, aiseats.SeatTracker, is already exposed through the
AGPL coderd.API as an interface). Moving it does not OSS any
license-gated feature: the /api/v2/aibridge HTTP route stays registered
only by enterprise/coderd and remains gated by
RequireFeatureMW(codersdk.FeatureAIBridge).

Net effect by build:
- AGPL: daemon starts when CODER_AIBRIDGE_*_ENABLED; the HTTP route is
  never registered, so curl /api/v2/aibridge returns 404.
- Unlicensed enterprise binary: daemon starts; HTTP route returns 403
  via RequireFeatureMW.
- Licensed enterprise: unchanged.

Files moved (with import-path rewrites):
- enterprise/aibridged              -> coderd/aibridged
- enterprise/aibridgedserver        -> coderd/aibridgedserver
- enterprise/coderd/aibridged.go    -> coderd/aibridged.go
  (RegisterInMemoryAIBridgedHTTPHandler and CreateInMemoryAIBridgeServer
   become methods on the AGPL API)
- enterprise/cli/aibridged.go       -> cli/aibridged.go
  (buildProviders is exported as BuildProviders so enterprise/cli can
   still build the proxy daemon's provider list; TestDomainsFromProviders
   moves with the proxy to enterprise/cli/aibridgeproxyd_internal_test.go)

The aibridged integration test stays enterprise-only since it uses
coderdenttest; it moves to enterprise/aibridged_integration_test.go.

In-place edits:
- enterprise/cli/server.go drops the bridge-daemon start block and the
  outdated TODO about lifecycle being enterprise-owned; only the proxy
  daemon start remains here. cli/server.go (AGPL) now constructs the
  bridge daemon next to the env-seed step.
- enterprise/coderd/coderd.go drops the aibridgedHandler field
  (now on the AGPL API) and reads it through api.AGPL.GetAIBridgedHandler.
- enterprise/coderd/aibridge.go reads api.AGPL.GetAIBridgedHandler().

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>