feat(server): accept host process runners

[chicoxyzzy]

What changed

• Exports a server constructor that accepts a host-provided process runner.
• Reports inline-image and embedded-context as unsupported so hosts use baseline ACP resource links for staged inputs.
• Uses the merged toolkit contract for sanitized environments, pre-cancel no-launch behavior, attachment-history privacy, and complete Unix/Windows provider process-tree containment.
• Updates package documentation for library consumers.

Why

Hecate needs to run this owned ACP server in-process while continuing to launch and supervise the vendor CLI as a child process with an exact executable path, sanitized environment, and bounded descendant lifecycle.

Impact

The standalone adapter binary keeps its existing launch and runtime behavior; its advertised optional prompt capabilities now accurately remain unsupported. Library consumers can own process launch policy without duplicating provider-specific session or protocol handling.

Validation

• go test ./...
• go vet ./...
• go test -race -timeout 10m ./...
• Windows test cross-build
• Authenticated live smoke through Hecate's embedded runtime

Rebased onto current main; toolkit #28 is merged and pinned at canonical revision 8510f612270d.