* Add zerostack provider scaffold
Register zerostack (gi-dellav/zerostack) as a core provider reading
plain-text sessions from $XDG_DATA_HOME/zerostack/sessions/. Parser is
modeled on pi.ts; on-disk schema is unverified and must be confirmed
against real sessions before a PR (see docs/providers/zerostack.md).
* Rework zerostack provider against real session schema
Replace the initial scaffold (modeled on pi.ts JSONL) with the verified
format from a real local run and the zerostack source:
- Sessions are one JSON file per session under the platform data dir
(~/Library/Application Support/zerostack/sessions on macOS,
$XDG_DATA_HOME/zerostack/sessions on Linux; ZS_DATA_DIR overrides).
- Tokens are cumulative session totals, not per-call, so emit one
ParsedProviderCall per session from total_input/output_tokens.
- Recompute cost via LiteLLM (calculateCost); OpenRouter ids arrive
route-prefixed and resolve through canonical names.
- zerostack persists only final assistant text, so tools/bash are empty.
Verified against a real session (DeepSeek v4 Pro via OpenRouter):
34.1K in / 961 out / $0.016, matching the recorded total_cost. Adds a
fixture-based test and rewrites the provider doc to match.
