Implement per-user chat threads (multiple conversations)

[bradvin]

Problem

Chat history is currently keyed only by user (clawpress_chat_history_<user_id>), so each user has exactly one conversation timeline. This blocks thread-based workflows.

Current State

• History persistence: includes/helpers/class-chat-history-helper.php.
• Chat API: includes/rest/class-chat-controller.php (/chat/message, /chat/history).
• Panel state tracks last_history_id, but no thread concept.

Proposed Data Model

Introduce per-user thread IDs and thread metadata:

• Option key for thread list/meta per user (or user meta), e.g. clawpress_chat_threads_<user_id>.
• History key becomes thread-scoped, e.g. clawpress_chat_history_<user_id>_<thread_id>.
• Thread metadata fields:
  • id, title, created_at, updated_at, archived.

Implementation Plan

1) Chat history helper refactor

• Add thread-aware APIs:
  • get_history_items( ?int $user_id = null, ?string $thread_id = null )
  • append_history_message(..., ?string $thread_id = null )
  • clear_history_items(..., ?string $thread_id = null )
• Maintain backward compatibility by defaulting to a default thread when thread ID not provided.

2) New thread helper

• Add helper for thread CRUD/list/select/archive, with sanitized thread IDs and title generation.

3) REST API updates

• Extend chat endpoints:
  • POST /chat/message accepts thread_id.
  • GET /chat/history accepts thread_id.
• Add thread endpoints:
  • GET /chat/threads
  • POST /chat/threads
  • POST /chat/threads/{id} (rename/archive/select metadata update).

4) Panel updates

• Add thread selector in panel.
• New message sends include active thread_id.
• Persist last active thread in panel state.

5) Migration

• On first use, seed default thread and map existing single history into that thread.
• No data loss migration path for existing users.

Acceptance Criteria

• A user can create and switch between multiple threads.
• Message history is isolated per thread.
• Existing users retain prior history in default thread.
• Thread switching does not leak tool events/history across threads.

Test Plan

• Unit tests for thread CRUD helper.
• Unit tests for thread-scoped history persistence.
• REST tests for thread endpoints and thread_id handling.
• Panel integration tests for switching and send behavior.

Out of Scope

• Cross-thread semantic search.
• Shared threads across users.