enhancement: /read endpoint must support ACP transcript data (not just JSONL)

[OneStepAt4time]

Description

From E2E testing + Hep investigation (2026-05-10): The /read endpoint (GET /v1/sessions/:id/read) only reads JSONL transcript files (tmux-era). In ACP mode, transcript data is stored differently and no JSONL file exists, so /read returns messages: [].

Meanwhile /transcript may partially work for ACP sessions through a different code path.

Root Cause

• /read → readTranscript → getCachedEntries → reads session.jsonlPath
• In ACP mode, session.jsonlPath is undefined → returns empty array
• ACP stores transcript data via its own transcriptId in the ACP session service
• No bridge between ACP transcript data and the /read endpoint

Impact

• Users on ACP mode (default) get empty responses from /read
• Docs reference /read as the primary way to read session output
• Inconsistency between /read and /transcript behavior

Proposed Fix

1. Detect ACP sessions and read from ACP transcript service instead of JSONL
2. Normalize output format to match existing /read response schema
3. Consider unifying /read and /transcript endpoints long-term

Acceptance Criteria

• /read returns messages for ACP sessions
• Output format consistent with JSONL-based reads
• Backward compatible with tmux sessions

Environment

• Aegis: v0.6.6-preview.1 (develop)
• Node: v22

[aegis-gh-agent]

Resolved by PR #3151 (merged on 2026-05-10). /read endpoint now falls back to ACP event store for ACP sessions.