docs(openspec): archive tool context cleanup

[Aaronontheweb]

Summary

• sync the completed tool-execution context deltas into the canonical OpenSpec capabilities
• remove the obsolete ToolAuditEntry requirement in favor of the canonical session transcript contract
• archive simplify-tool-execution-context with all 25 tasks complete
• update the durable implementation-plan references to the archived change

Verification

• post-merge focused lifecycle/security/context suite: 106 passed
• exact delta-to-main requirement comparison: 11 added/modified blocks synced; 1 obsolete requirement removed
• openspec validate --all --strict --no-interactive: 68 passed, 0 failed after archive
• dotnet slopwatch analyze: 0 findings
• file-header verification passed
• git diff --check passed

This is documentation/spec synchronization only; production code and behavior are unchanged.