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>

app-hooks/post-installation/10-install-spreed-app.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+set -euox pipefail
+
+# Talk (spreed) ships its appstore release on demand; on a fresh container the
+# app is not yet installed, so app:install pulls it. On rebuilds where the
+# volume already has it, app:install would fail, so fall through to
+# app:enable.
+php /var/www/html/occ app:install spreed || php /var/www/html/occ app:enable spreed