feat(api): session export endpoint — download transcript as JSONL/Markdown (#3114)

[OneStepAt4time]

Summary

Implements #3114

Adds GET /v1/sessions/:id/export?format=jsonl|markdown to download full session transcripts.

Changes

• src/routes/session-data.ts: New /v1/sessions/:id/export route with two formats:
  • format=jsonl — NDJSON stream, one entry per line, with Content-Disposition: attachment
  • format=markdown — Readable markdown export with section headers, thinking blocks (💭), tool calls in collapsible <details>, permission requests (🔐), and error callouts (⚠️)
• Returns 400 for unsupported formats, 404 for missing sessions or empty transcript
• Auth/ownership via withOwnership middleware (tenant isolation)
• 9 tests covering all formats, edge cases, and markdown rendering

Verification

tsc --noEmit: ✅ 0 errors
npm run build: ✅ OK
npm test: ✅ 9/9 passed

Example Markdown output

# Session Export: my-session

> Exported: 2026-05-10T15:50:00Z
> Session ID: abc-123

---

### 👤 User
Hello, how are you?

### 💭 Thinking
Let me think about this...

### 🤖 Assistant
I am doing well, thanks!

### 🔧 Tool: ReadFile
<details>
<summary>ReadFile</summary>
file contents here
</details>

[aegis-gh-agent]

✅ Approved — all 9 merge gates passed (pending bump gate re-run).

Session export endpoint — JSONL + Markdown formats. 10 tests. Content-disposition for download. All CI green.

👁️ Argus