fix(antigravity): serve the transcript trace when a session has no .db

[VasiHemanth]

Follow-up to #229.

The problem

#229 taught _antigravity_cli_trace to fall back to brain/<sid>/.system_generated/logs/transcript*.jsonl when a session has no conversations/<sid>.db — newer agy builds no longer write one. That half works.

But get_session_detail only called it when the .db existed:

cli_db = ANTIGRAVITY_CLI_DIR / "conversations" / f"{session_id}.db"
if cli_db.exists():                       # <- fallback never runs without a .db
    cli_msgs = _antigravity_cli_trace(cli_db, session_id)

So the fallback never ran for exactly the sessions it was written for. All 29 sessions #229 restores have no .db; they dropped through to the brain-markdown branch, and 26 of them rendered nothing at all. The bug hwantage reported was still on screen after the merge.

This was my miss reviewing #229 — I validated _antigravity_cli_trace(db, sid) directly, which exercises a fallback the endpoint never reached.

The fix

_antigravity_cli_trace already does if db_path and db_path.exists() internally, so the outer guard was redundant as well as harmful. Drop it and let the function decide.

Measured

Over the 167 Antigravity brain sessions on this machine, through the running API:

sessions returning zero messages : 59   -> 35
total messages served            : 6724 -> 7814   (+1090)
kind=antigravity_cli             : 62   -> 91

Spot-check on two sessions that previously rendered nothing:

615e6853  kind=antigravity_brain  messages=1    ->  kind=antigravity_cli  messages=168
97fe56ed  kind=antigravity_brain  messages=2    ->  kind=antigravity_cli  messages=164

No session loses events. The guard only ever affected sessions without a .db, and those previously always fell through to brain markdown; the arithmetic closes exactly (6724 + 1090 = 7814).

The 35 still returning zero have neither a usable transcript nor a .db — pre-existing and out of scope here.

Test

test_session_detail_uses_transcript_trace_when_there_is_no_sqlite_db drives get_session_detail, not the parser, since the parser was never the broken half. It also plants a task.md so it pins the precedence (real trajectory beats synthesized markdown).

Against the prior code it fails with AssertionError: fell through to 'antigravity_brain'. With the fix, test_antigravity_cli.py is 13 passed. Full backend suite: 382 passed, same 23 pre-existing environment failures as main.

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