docs: fix stale exec plan path

[goanpeca]

Summary

• Point active work-plan references at the existing docs/exec-plans/ directory.
• Remove the stale docs/exec-plans/active/ reference from AGENTS.md.

Linked issue

Closes #1

Tests run

• mamba run -n docs-to-podcast-issue-1 node -e "const fs=require('fs'); const doc=fs.readFileSync('AGENTS.md','utf8'); if (doc.includes('docs/exec-plans/active/')) throw new Error('stale active exec-plan reference remains'); if (!fs.statSync('docs/exec-plans').isDirectory()) throw new Error('docs/exec-plans missing');"
• git diff --check

Follow-up notes

• No follow-up needed.