fix(agent): stop resume hydration from overfilling the context window

[tatoalo]

Problem

Resuming a long-lived cloud task can fail hard with Internal error: Prompt is too long, and once it does, every subsequent resume of that task fails the same way so the task is permanently stuck

Loosely:

1. Directory resume snapshots only cover the workspace, not ~/.claude, so the native session transcript is never on disk after a resume (warm: false). Every resume goes through hydrateSessionJsonl, which rebuilds a transcript from the task's ACP notification logs.
2. The logs endpoint concatenates the entire resume chain (oldest ancestor first), and in-session compactions are invisible in that log, so hydration resurrects history the session had already compacted away.
3. selectRecentTurns budgets 800k estimated tokens for 1M-context models at 4 chars/token. JSON-heavy tool payloads actually tokenize at ~2.5–3 chars/token, so the "800k" selection can be well over 1M real tokens. In the failing run, hydration kept 66 turns (~2.9M chars, estimated 732k tokens) that the API counted as 1,155,179 tokens against a 1,000,000 window. The 200k-window budget (150k estimated) has the same overflow, just less headroom to hide it.
4. The rebuilt transcript writes usage: 0 on every message, so the SDK's context accounting starts near zero: it can't auto-compact before the first API call (the run's compact_boundary reported preTokens: 16250 while getContextUsage() said 1.15M). The first call 400s, the SDK's recovery compaction can't help, and the turn fails as a non-recoverable agent_error.

Changes

• Estimate turn cost at 3 chars/token instead of 4, so JSON-heavy payloads are no longer undercounted
• Lower the hydration budgets to target roughly half the real window: 400k estimated for 1M-context models (worst case ~500k real), 80k for 200k-window models. This leaves room for system prompt, tools, skills, and the resumed run's own work, and un-bricks existing tasks (the next resume re-hydrates within budget)

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit ec17957.}