docs: retire completed remediation plans

[attmous]

Summary

• remove the completed architecture remediation backlog and S01-S20 execution plan
• remove the superseded pre-modular structure review and Coordinator handoff
• refresh living architecture, README, and agent references so the July audit is historical evidence rather than an active queue
• update the workflow contract to preserve completion provenance in the retained audit

Verification

• scripts/dev/test-dev-workflow.ps1 -SelfTest
• scripts/dev/verify-musfit.ps1 -Preset Full -RetryOnGeneratedOutputIssue
• changed-document relative link scan
• git diff --check

Device evidence

Not applicable: documentation and documentation-consistency tooling only; no runtime UI or behavior changed.

Risk and rollback

Low documentation-only risk. Restore the four deleted files and their references to roll back, though they contain expired instructions from the completed July 2026 remediation program.