feat: expose turn-start metadata to extensions

[jif-oai]

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

[jif-oai]

@codex review

[chatgpt-codex-connector]

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