docs: correct loop-control subsystem descriptions in ARCHITECTURE.md/PRODUCT.md (#620)

* docs: correct ARCHITECTURE.md and PRODUCT.md for the deleted loop-control subsystem

repetition.ts, period-detection.ts, the tool-fingerprint thrash/backstop
checks, requireEdit, never-acted/never-edited/no-progress/repetition stop
classes, the hard-block re-dispatch refusal, and the default 30-turn leaf
cap are gone. Describe the leaf stops, the fleet dispatch ledger, the
task-tool watchdog exemption, and the primary-session nudge as they
actually behave today instead of citing deleted files and thresholds.

* docs: drop the stale hard-cap-4-workers prompt description

CL-7001 is removing the same claim from the actual prompt text
(src/agent/prompts.ts, skywalker/package.ts) since no concurrency cap
exists in code. Removing it here too so the two land consistent.

Author: TheGreatAxios