feat: add native macOS iMessage channel

[cherryFloris]

Summary

Implements the iMessage channel requested in #137, using macOS Messages.app
directly as the transport; no BlueBubbles or third-party gateway required.

Included

• Receive / send text messages over the native Messages.app transport
  (~/Library/Messages/chat.db for receive, /usr/bin/osascript for send)
• Follows the existing channel adapter and runtime architecture; iMessage bot,
  workspace, contact mapping, and Harness sessions stay isolated
• One iMessage identity per macOS user account, each contact mapped to an
  independent Harness session
• Host registration and management RPC support (/api/dsh-im/imessage)
• iMessage settings UI with local permission checks; Full Disk Access and
  Automation permission guidance
• Credentials, permissions, cursors, and runtime state stored locally on the
  Host; no sensitive data returned through browser-facing RPC
• Delivery adapter and proactive text delivery support
• iMessage session title prefixes and session list logos
• Docs for installation, permissions, testing, security boundaries, and
  limitations

MVP scope (as outlined in #137)

• Text message receive / send
• Direct / private conversations
• Independent Harness session per contact
• Native macOS Messages.app transport

Intentionally out of scope (first release)

Group chats, images, files/attachments, rich text, quoted replies, reactions,
message edit/recall, multiple Messages.app accounts, and remote
BlueBubbles-style operation.

Permissions and security

Users must explicitly grant:

1. Full Disk Access to the terminal / app running DeepSeek Harness
2. Automation permission to control Messages.app

chat.db is only read via read-only queries; messages are sent only through
the native Messages.app automation interface.

Compatibility notes

• macOS-specific; chat.db is an internal database rather than a stable public
  API, so future macOS releases may require compatibility updates
• For testing use a separate iMessage address; self-chat may cause message
  loops or duplicate replies

Testing

• npm run check passes
• 2590 automated tests passed
• Package artifact verification passed
• Session channel logo browser verification passed

Manual flow:

1. Sign in to iMessage in Messages.app
2. Grant Full Disk Access and Automation permissions
3. Enable iMessage in the dsh-im settings page
4. Send a text from another phone number / Apple ID
5. Verify Harness receives the message and replies via Messages.app
6. Verify the session is labeled with the iMessage channel identity