fix(subagents): coalesce child refreshes without losing observations

[Juliusolsson05]

Parent JSONL bursts and periodic polls could read the same child byte range concurrently, duplicating turns/tool calls and emissions (#802). Each Claude/Codex tracker now has one refresh owner and coalesces requests received during I/O into a trailing pass. Late appends and parent completion still drain; stopped trackers cannot repopulate offsets/accumulators or emit after a pending read completes. Transient poll errors remain retryable without unhandled background rejections.

Validation:

• 34 focused subagent tests pass, including 10 new behavioral cases across both providers: overlap, late append/completion, split UTF-8 JSONL, transient error, and stop during I/O.
• The same controlled burst tests fail against main: up to 52 concurrent range reads before, exactly one after; a late append requires two sequential reads. This is synthetic evidence, not a production CPU/FPS claim.
• npm run typecheck, npm run test:contract, and git diff --check pass.
• Reviewed refresh ownership, promise settlement, stop boundaries and per-provider correlation. Both full CI gates pass after the main rebase.

Plan committed before implementation: docs/superpowers/plans/2026-09-05-subagent-refresh-coalescing.md. No settings/protocol migration. Missing-rollout discovery remains separate (#803); follow-up #811 explicitly depends on this branch. No running agents or user working-tree files were changed.

Fixes #802

Current validation: rebased onto main f7507980 (toolkit #812 and MCP repair #818), head 51f47af4. Source changes from the earlier implementation are preserved; both required CI gates pass on this head (CI run). No merge is authorized.