fix(antigravity): read token usage from the conversation, not the Step

[uipreliga]

Summary

google-antigravity 0.1.18 (bumped in #196, 3aa2db30) deprecated Step.usage_metadata. Usage is now billed per model invocation on the conversation (conversation.total_usage), and the per-step field is always None. The adapter still read the per-step field. The result, since 0.12.7:

• Every Antigravity turn recorded no tokens and no cost (total_token_usage: null).
• max_turns never tripped on Antigravity. api_calls is incremented when a new MODEL step opens a call, and the call is closed only by a usage-bearing step. With no usage, the first call never closed, so the count stayed at 1.
• Nothing failed loudly. make verify stayed green, because the test fakes still put usage on the Step.

Fix

• _AntigravityTurnState takes a cumulative_usage source (the conversation's total_usage). It books each rise since the last reading as one generation, and books any remainder at finalize. So the per-message buckets still sum to the turn total, and the reconciliation invariant holds.
• The per-step read is removed completely. The SDK pin is 0.1.18, so there is no fallback to maintain.
• The test fakes now bill through a _UsageMeter, the same way the SDK connection accumulates usage, and no longer put usage on the Step. The existing scenarios therefore exercise the new source.
• Two regression tests fail on the old code:
  • a Step that still carries usage_metadata books nothing;
  • usage billed after the last Step is booked at finalize, with the correct call count.
• The notes and docs that described usage "on a Step" are updated (.claude/notes/timing.md, docs/agents/HARNESS_PARITY.md).
• A guardrail candidate is added to .claude/harness-candidates.md: a static rule cannot see an SDK field go dead, but a one-turn live-telemetry smoke in the harness-bump checklist would have.

Verification

• make verify: pass (6101 passed; format, lint, pyright, custom rules, docs budget).
• Live run, hello_date on gemini-3.8-flash, before and after the fix:
  • before: total_token_usage: null, num_turns: 1
  • after: 30,774 input / 803 output tokens, 3 generations, num_turns: 3, $0.052 at list price
• Also exercised in a 15-row, five-agent run (gemini-3.8-flash, 3 repeats): every Antigravity row has tokens and cost.

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