feat: harden degraded inspection and config validation

[ganesh47]

Summary

• validate malformed active config surfaces with source-aware, path-specific errors
• make stale child lineage more visible in inspect summaries and suggestions
• add degraded-run coverage for stale child runs and malformed run-ledger entries

Why

This is the first execution slice from the long-horizon spec-gap closure board. It targets the highest-leverage trust gaps from the audit:

• malformed config silently undermining workflow behavior
• partial or stale run artifacts making inspect less trustworthy
• malformed run directories weakening the ledger surface

Validation

• npm test -- test/config.test.ts test/inspect.test.ts test/runs.test.ts
• npm run typecheck
• npm test
• npm run build
• /tmp/sqlite-metadata-proposal:
  • bin/cstack.js runs --recent 3
  • bin/cstack.js inspect 2026-03-16T12-49-00-888Z-intent-what-are-the-gaps-in-this-project-can-you-work-o

Notes

• This PR intentionally does not tackle the broader GitHub-robustness track yet.
• Unknown config keys are still tolerated; this slice hardens known active config surfaces first.