feat(run): InputFileStager seam — stage non-image attachments into the prompt

executus's tool.Invocation already carried InputFiles (audio/PDF/binary), but the
executor never staged them — only Images were folded into the run. This adds the
host seam mort's chat/chatbot surfaces need for audio-input parity with agentexec.

- run.Ports gains InputFiles InputFileStager (nil-safe; nil = input files silently
  ignored, run still proceeds text-only). The interface mirrors mort's skill
  FileStorage: StageInputFile(ctx, runID, agentID, name, mime, content) → file_id.
- run/input_files.go (ported from mort agentexec/input_files.go): stageInputFiles
  persists each file under run scope and appends an [ATTACHED FILES] descriptor
  block to the prompt so the agent can reach them by file_id (e.g. code_exec
  files_in → /workspace/<name>). Bytes are NEVER inlined into model context.
  Best-effort: empty/oversized(>50MB)/save-error files are skipped; colliding
  base names are disambiguated (name-2, name-3) so they don't clobber at
  /workspace/<name>.
- Executor.Run calls it after the model/toolbox build, before the loop, so the
  descriptor rides the first user turn (alongside the existing Images folding).

Tests: stages + builds the block; nil stager / no files leave the prompt intact;
dedup; empty/save-error skipping. Full suite green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

run/ports.go

@@ -42,6 +42,20 @@ type Ports struct {
 	// Delivery is where the run's output + artifacts go. nil = the caller
 	// reads the Result in-process (the light-host default).
 	Delivery deliver.Delivery
+	// InputFiles persists non-image input attachments (audio, PDF, binary)
+	// carried on Invocation.InputFiles into a host file store under run scope,
+	// returning file_ids the agent can hand to a worker tool. nil = input files
+	// are silently ignored (the run still proceeds, text-only). The bytes are
+	// never inlined into the model context — the LLM can't read raw audio/binary.
+	InputFiles InputFileStager
+}
+
+// InputFileStager persists a single non-image input attachment into a host file
+// store under run scope and returns a file_id the run can reference. It is the
+// seam mort's skill FileStorage (and any host blob store) implements so the
+// kernel can stage Invocation.InputFiles without importing a storage layer.
+type InputFileStager interface {
+	StageInputFile(ctx context.Context, runID, agentID, name, mime string, content []byte) (fileID string, err error)
 }
 
 // RunInfo describes a run at start time — the attribution a recorder/critic