feat(gateway): deliverable mode — ship artifacts as native uploads from any agent surface (#27813)

The agent can now produce a chart, PDF, spreadsheet, or any other supported
file type and have it land in Slack / Discord / Telegram / WhatsApp / etc.
as a native attachment, just by mentioning the absolute path in its
response. Same primitive works for kanban-worker completions: workers
attach artifacts via kanban_complete(artifacts=[...]) and the gateway
notifier uploads them alongside the completion message.

Changes:

- gateway/platforms/base.py: extract_local_files now covers PDFs, docx,
  spreadsheets (xlsx/csv/json/yaml), presentations (pptx), archives
  (zip/tar/gz), audio (mp3/wav/...), and html — not just images and video.
  Image/video extensions still embed inline; everything else routes to
  send_document via the existing dispatch partition in gateway/run.py.

- tools/kanban_tools.py + hermes_cli/kanban_db.py: kanban_complete gains
  an explicit ``artifacts`` parameter. The handler stashes it in
  metadata.artifacts (for downstream workers) and the kernel promotes
  it onto the completed-event payload so the notifier can find it
  without a second SQL round-trip.

- gateway/run.py: _kanban_notifier_watcher now calls a new helper
  _deliver_kanban_artifacts after sending the completion text. The
  helper reads payload.artifacts (preferred), falls back to scanning
  the payload summary and task.result with extract_local_files, then
  partitions images / videos / documents and uploads each via
  send_multiple_images / send_video / send_document.

- website/docs/user-guide/features/deliverable-mode.md + sidebars.ts:
  user-facing docs page covering the extension list, the kanban
  artifacts pattern, and the MCP-for-connector-breadth recommendation.

Tests:

- tests/gateway/test_extract_local_files.py: 7 new test cases
  (documents, spreadsheets, presentations, audio, archives, html,
  chart-pdf canonical case). 44 passing, 0 regressions.
- tests/tools/test_kanban_tools.py: 4 new cases covering the artifacts
  arg shape (list / string / merge with existing metadata / type
  rejection). 17 passing.
- tests/hermes_cli/test_kanban_notify.py: 2 new cases covering full
  notifier → artifact-upload path and missing-file silent-skip. 12
  passing.
- E2E (real files, real kanban kernel, real BasePlatformAdapter):
  worker calls kanban_complete(artifacts=[png,pdf,csv]) → metadata +
  event payload land → notifier helper partitions correctly →
  send_multiple_images called once with the PNG, send_document called
  twice with PDF + CSV.

What's NOT in this PR (deferred to follow-ups):

- Ad-hoc "research this for two hours, ping the thread when done"
  slash command — covered today by kanban subscriptions; a dedicated
  slash command can ride a follow-up PR if needed.
- Setup-wizard prompt for recommended MCP servers (Notion, GitHub,
  Linear, etc.) — docs page lists them; UI is a separate change.

Plan and rationale captured in ~/.hermes/docs/perplexity-computer-parity.pdf
(local doc, not shipped).

hermes_cli/kanban_db.py

@@ -2479,6 +2479,20 @@ def complete_task(
         }
         if verified_cards:
             completed_payload["verified_cards"] = verified_cards
+        # Carry artifact paths in the event payload so the gateway
+        # notifier can upload them as native attachments alongside the
+        # completion message. Workers pass these via
+        # ``kanban_complete(artifacts=[...])`` which stashes the list in
+        # ``metadata["artifacts"]`` — we promote it onto the event so
+        # consumers don't have to fetch the run row to find it.
+        if isinstance(metadata, dict):
+            md_artifacts = metadata.get("artifacts")
+            if isinstance(md_artifacts, (list, tuple)):
+                cleaned_artifacts = [
+                    str(p).strip() for p in md_artifacts if isinstance(p, str) and str(p).strip()
+                ]
+                if cleaned_artifacts:
+                    completed_payload["artifacts"] = cleaned_artifacts
         _append_event(
             conn, task_id, "completed",
             completed_payload,