feat(kanban): attachment toolset + CLI to match the dashboard surface

The kanban board has had full attachment storage and a dashboard HTTP
API (upload/list/download/delete) since #35338, but there was no agent
toolset tool and no `hermes kanban` CLI verb for attachments. Agents and
scripts that don't go through the dashboard server (or can't touch the DB
directly) had no way to create or read real attachments — only links in
comments.

Close that gap by mirroring the existing comment surface:

- `kanban_db.store_attachment_bytes()` — one shared write path (validate
  name, enforce the 25 MB cap, write the blob under the per-task dir with
  collision-free naming, insert the metadata row, clean up an orphan blob
  if the insert fails). `_MAX_ATTACHMENT_BYTES`, `_safe_attachment_name`,
  and a new `_collision_free_path` move here so the dashboard, the tool,
  and the CLI all share one implementation and can't drift.
- Tools (`tools/kanban_tools.py`): `kanban_attach` (inline base64),
  `kanban_attach_url` (server-side http/https fetch with the same cap),
  `kanban_attachments` (list). Write tools respect worker task-ownership;
  list is read-only. Registered in the `kanban` toolset.
- CLI (`hermes_cli/kanban.py`): `attach <id> <path>`, `attachments <id>`,
  `attach-rm <attachment_id>`.
- Dashboard `upload_task_attachment` now imports the shared helpers and
  uses `_collision_free_path` — behavior identical (still streams to disk
  with the cap, still 413 on overflow).
- Docs (AGENTS.md, kanban-worker skill) and toolset membership updated.

Tests: tool round-trip + oversize + bad base64 + ownership; attach_url
against a local HTTP fixture incl. oversize-mid-stream and non-http
scheme rejection; CLI attach/attachments/attach-rm; shared-helper unit
tests; dashboard parity preserved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

toolsets.py

@@ -75,6 +75,7 @@ _HERMES_CORE_TOOLS = [
     "kanban_complete", "kanban_block", "kanban_heartbeat",
     "kanban_comment", "kanban_create", "kanban_link",
     "kanban_unblock",
+    "kanban_attach", "kanban_attach_url", "kanban_attachments",
     # Computer use (macOS, gated on cua-driver being installed via check_fn)
     "computer_use",
 ]
@@ -264,14 +265,15 @@ TOOLSETS = {
             "set). The dispatcher runs inside the gateway by default; see "
             "`kanban.dispatch_in_gateway` in config.yaml. Lets workers mark "
             "tasks done with structured handoffs, block for human input, "
-            "heartbeat during long ops, comment on threads, and (for "
-            "orchestrators) list, unblock, and fan out tasks."
+            "heartbeat during long ops, comment on threads, attach files, and "
+            "(for orchestrators) list, unblock, and fan out tasks."
         ),
         "tools": [
             "kanban_show", "kanban_list", "kanban_complete", "kanban_block",
             "kanban_heartbeat", "kanban_comment",
             "kanban_create", "kanban_link",
             "kanban_unblock",
+            "kanban_attach", "kanban_attach_url", "kanban_attachments",
         ],
         "includes": [],
     },