Fixes: coder/internal#1441
- Move `contextConfigAPI` init from `handleManifest` to `init()`,
matching all other API fields
- Change `agentcontextconfig.NewAPI` to accept `func() string` closure
(lazy directory evaluation)
- `Config()` and HTTP handler now compute on demand via
`a.manifest.Load().Directory`
- Widen `TestAgent_Reconnect` to loop 5 reconnections with a non-empty
manifest directory
- Add `TestContextConfigAPI_InitOnce` internal test verifying lazy eval
across manifest changes
- Add `TestNewAPI_LazyDirectory` unit test for the lazy contract
> 🤖 Written by a Coder Agent. Reviewed by a human.
Cian Johnston
cd784c755a