refactor: rename cmd/charon → cmd/responsed, add cmd/proxyd, cmd/conversationd stubs

[elevran]

Scope

Structural rename and stub creation — no new functionality. The current cmd/charon binary embeds both the Charon storage/resolution service and an optional proxy layer in a single binary controlled by --proxy. This issue splits that into three purpose-named binaries:

Binary	Description
cmd/responsed	Current Charon behaviour: internal storage/resolution service (/responses, /staging, /healthz, /readyz)
cmd/proxyd	Current cmd/proxy: client-facing Responses API proxy
cmd/conversationd	Stub only — compiles and starts, returns 501 Not Implemented on all /conversations routes

The combined single-binary mode (proxy + charon in one process for testing) is preserved by building cmd/responsed with an embedded cmd/proxyd handler, gated by a --proxy flag, matching the existing --proxy behavior in the current binary.

Changes required

• Rename cmd/charon/ → cmd/responsed/; update all internal import paths and package names
• Extract cmd/proxy/ handler registration logic and wire it as a standalone main in cmd/proxyd/
• Create cmd/conversationd/ stub: compiles, logs startup, returns 501 on POST /conversations, GET /conversations/{id}, DELETE /conversations/{id}, PUT /conversations/{id}/messages
• Update Makefile: add targets build-responsed, build-proxyd, build-conversationd, build-all; preserve existing build target pointing at cmd/responsed
• Add stub Dockerfile.responsed, Dockerfile.proxyd, Dockerfile.conversationd (copy from existing Dockerfile, adjust binary names)
• Update cmd/responsed/main.go to embed cmd/proxyd handler in combined mode

Acceptance criteria

• go build ./cmd/responsed/... produces a binary that behaves identically to today's cmd/charon
• go build ./cmd/proxyd/... produces a binary that behaves identically to today's cmd/proxy
• go build ./cmd/conversationd/... compiles; all /conversations routes return 501
• All existing tests pass (no behaviour change)
• make presubmit passes
• Each binary has a dedicated Makefile target and Dockerfile stub