fix(agent): seed hydrated transcripts with estimated context usage

[tatoalo]

Problem

When a cold resume rebuilds the session transcript from S3 logs, every rebuilt message carries usage: 0. The resumed SDK session therefore believes its context is nearly empty: it cannot trigger auto-compaction proactively and only learns the real context size from the first API response — or from a "prompt is too long" rejection, at which point compaction itself is already impossible. This was visible in a real failure where the SDK reported a compact boundary of ~16k pre-tokens while the actual request weighed over 1.1M tokens.

Follow-up to #3331.

Changes

• After building the hydrated JSONL, seed the last assistant message's usage.input_tokens with the estimated token count of the rebuilt conversation, so the SDK's context accounting starts near reality and auto-compaction can fire before the first API call when warranted.

How did you test this?

• New test: only the last assistant entry carries the non-zero estimate; earlier entries stay at zero.
• pnpm exec vitest run src/adapters/claude/session/jsonl-hydration.test.ts — 58 passed, 1 pre-existing failure (sanitizeSessionJsonl warn-path test) that also fails on main.
• tsc --noEmit on the agent package.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 03f7aa9.}

[tatoalo]

with the hydration budgets capped at ~half the window (#3331) and the workspace transcript mirror making cold hydration rare (#3333), the SDK learns its real context size from the first API response anyway, so seeding estimated usage adds little. The prompt-too-long fallback (#3334) covers the residual risk.