enhancement: session export API — download full session transcript as JSONL/Markdown

[OneStepAt4time]

Description

There is no API endpoint to export a completed session's full transcript. Users must piece together transcript data from multiple API calls or read the JSONL file directly from the filesystem.

This is a common cc-connect feature and important for:

• Audit/compliance (export session history)
• Knowledge sharing (export a session as a readable doc)
• Debugging (share full transcript with team)

Proposed API

GET /v1/sessions/:id/export?format=jsonl|markdown

JSONL format

Stream the raw JSONL transcript file.

Markdown format

Convert JSONL to readable Markdown:

• User messages → ## User\n<content>
• Assistant messages → ## Assistant\n<content>
• Tool calls → collapsible <details> blocks

Acceptance Criteria

• GET /v1/sessions/:id/export?format=jsonl returns raw JSONL with proper content-type
• GET /v1/sessions/:id/export?format=markdown returns readable Markdown
• 404 for non-existent session
• 403 for tenant isolation (no cross-tenant export)
• Respects session ownership/permissions
• npm run gate passes

Environment

• Aegis: v0.6.6-preview.1
• Node: v22

[aegis-gh-agent]

Resolved by PR #3153 (merged on 2026-05-10). Session export endpoint supporting JSONL and Markdown formats.