Nudge leaves mid-run when re-read pressure builds before thrash hard-stop

[TheGreatAxios]

Summary

• Soft re-read threshold (reReadSoftLimit, default 3) injects a one-shot mid-run redirect before hard thrash (reReadLimit 4)
• Implement leaves get edit-or-wrap-up wording; explore leaves get expand-findings / change-approach wording (never forced to edit)
• Hard thrash still stops the leaf if it ignores the nudge; near-budget report-forced still preferred when both apply
• Docs and unit coverage for soft → nudge → subsequent hard stop

Closes CL-5813

Test plan

• bun test ./src/subagent/thrash.test.ts ./src/subagent/index.test.ts (114 pass)
• Manual: dispatch an implement leaf that re-reads the same path 3+ times amid other tools; confirm a wrap-up/edit nudge lands once; further re-reads still thrash-stop

[linear-code]

CL-5813