mcp: headersCommand missing from V2 ConfigMCP.Remote schema (merge drift risk)

Found during v0.9.1 release review (Tech-Lead). `packages/core/src/v1/config/mcp.ts` `ConfigMCPV1.Remote` has `headersCommand` (bearer-auth, #793). The newer Effect-Schema `ConfigMCP.Remote` in `packages/core/src/config/mcp.ts` does NOT. Not a live bug today (MCP load goes through V1), but V2 `Config.Service` is already wired into other subsystems. If MCP config loading migrates to V2, `headersCommand` will silently vanish via `onExcessProperty: "ignore"` — dropped auth headers, no error. Add `headersCommand` to V2, or decide V2 `ConfigMCP` is dead and remove it.