[Feature]: Add read-only thread export and history search to the T3 CLI #8555

Summary

Add a supported, read-only CLI for finding and exporting T3 Code conversations. Agents and users should not need to rediscover provider JSONL layouts or query the internal SQLite projection directly.

Problem

Common workflows need to recover an earlier thread, preview, requirement, or handoff. Today this often falls back to scanning provider transcripts with ad hoc shell scripts. Those scans can include worker copies, depend on missing utilities, and produce approximate counts.

Proposed behavior

Provide commands along these lines:

t3 history thread <uuid>
t3 history search <query>

Support lookup by thread UUID, title, project, and time. Output readable text by default and JSON on request.

Include:

• Ordered user and assistant messages with timestamps.
• Thread ID, exact title, project, and provider metadata.
• Attachment references.
• Relevant preview URLs, commits, and selected activity when available.
• Stable deduplication of worker or subagent copies.

Keep the command read-only and build it on a supported application service rather than exposing the projection database as public API.

Acceptance criteria

• Exporting a thread returns one canonical ordered conversation.
• Search works across project and time filters.
• JSON output is stable enough for agent tooling.
• The command never mutates thread or provider state.
• Missing or unsupported records fail without preventing other history from being searched.

Related: #1163 covers one-click transcript copying in the UI and explicitly excludes broader history management. This issue covers CLI search and agent retrieval.

Tags: enhancement, cli, history, agent-workflow