feat(talk): add MCP integration for Nextcloud Talk (spreed)

Adds 6 MCP tools so an LLM can read a user's Talk conversations and
post messages on their behalf, addressing the "read my chats and reply"
use case from issue #720:

  - talk_list_conversations
  - talk_get_conversation
  - talk_get_messages
  - talk_list_participants
  - talk_send_message    (auto-attaches a referenceId for retry dedup)
  - talk_mark_as_read

Edit/delete messages, reactions, threads, and call/session ops are
intentionally out of scope for this first PR.

The TalkClient also exposes create_conversation/delete_conversation
for the integration test fixture; these are not registered as MCP
tools. A post-installation hook enables spreed in the docker dev env
so the integration suite has a real Talk backend to talk to.

Closes #720

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/client/__init__.py

@@ -23,6 +23,7 @@ from .news import NewsClient
 from .notes import NotesClient
 from .sharing import SharingClient
 from .tables import TablesClient
+from .talk import TalkClient
 from .users import UsersClient
 from .webdav import WebDAVClient
 from .webhooks import WebhooksClient
@@ -85,6 +86,7 @@ class NextcloudClient:
         self.collectives = CollectivesClient(self._client, username)
         self.deck = DeckClient(self._client, username)
         self.news = NewsClient(self._client, username)
+        self.talk = TalkClient(self._client, username)
         self.users = UsersClient(self._client, username)
         self.groups = GroupsClient(self._client, username)
         self.sharing = SharingClient(self._client, username)