feat: add reply-to message threading

[bertheto]

Problem

Messages in AgentChatBus are purely sequential (identified by seq number only). There is no way for a message to explicitly reference a parent message, making it impossible to model reply chains, threaded discussions, or contextual annotations within a thread.

This is a universal messaging primitive present in all major platforms (Slack, Discord, GitHub comments, email threads).

Solution

Add an optional reply_to_msg_id field to the Message model. When set, it references a parent message that must exist in the same thread. The field is stored as a dedicated column (not in metadata) for queryability.

Changes

• src/db/models.py: reply_to_msg_id: Optional[str] = None added to Message dataclass
• src/db/database.py: additive migration ALTER TABLE messages ADD COLUMN reply_to_msg_id TEXT + index idx_messages_reply
• src/db/crud.py: safe fallback in _row_to_message() for older DBs; new msg_get() function; msg_post() validates parent exists and belongs to the same thread; emits msg.reply SSE event when reply_to_msg_id is set
• src/mcp_server.py: reply_to_msg_id property added to msg_post inputSchema
• src/tools/dispatch.py: handle_msg_post() passes/returns reply_to_msg_id; handle_msg_list() includes field; ValueError from validation returned as structured error
• src/main.py: MessageCreate.reply_to_msg_id; POST response and GET message list include the field
• src/static/index.html + src/static/css/main.css: visual reply-quote block (.msg-reply-quote) shown above message content when reply_to_msg_id is present
• tests/test_reply_threading.py: 18 tests (12 unit with in-memory DB + 6 integration against real server)

Backward Compatibility

• reply_to_msg_id defaults to null — all existing msg_post calls are unchanged
• Existing databases are handled by a safe column presence check in _row_to_message()
• The new field is additive in all API responses; existing consumers can ignore it

Validation Rules

• reply_to_msg_id must reference an existing message ID in the same thread
• Cross-thread references raise ValueError (HTTP 400 via REST, structured error via MCP)
• Non-existent message IDs raise ValueError (HTTP 400)

SSE Event

A new msg.reply event is emitted when a reply is posted:

{
  "type": "msg.reply",
  "payload": {
    "msg_id": "...",
    "reply_to_msg_id": "...",
    "thread_id": "...",
    "author": "agent-name",
    "seq": 5
  }
}