## Why
The goal extension needs more context when a turn starts than
`turn_store` alone provides.
In particular, goal accounting needs the stable turn id, the effective
collaboration mode, and the cumulative token-usage baseline captured at
turn start so it can:
- suppress goal accounting for plan-mode turns
- compute exact per-turn deltas from cumulative `total_token_usage`
snapshots instead of relying on the most recent usage event alone
- keep the extension-owned accounting path aligned with the host turn
lifecycle
## What
- extend `codex_extension_api::TurnStartInput` to expose `turn_id`,
`collaboration_mode`, and `token_usage_at_turn_start`
- pass the full `TurnContext` plus the captured token-usage baseline
through the turn-start lifecycle emission path
- initialize goal turn accounting from the turn-start baseline and
collaboration mode
- switch goal token accounting to compute deltas from cumulative
`total_token_usage` snapshots
- add coverage for the new turn-start lifecycle fields and for
goal-accounting baseline behavior
## Testing
- added `turn_start_lifecycle_exposes_turn_metadata_and_token_baseline`
in `codex-rs/core/src/session/tests.rs`
- added `ext/goal/tests/accounting.rs` coverage for baseline-aware goal
accounting and plan-mode suppression
