fix(tooling): migrate deprecated mise monorepo setting

[ayushtr-aws]

The reported security:deps failure was fixed on main by #870, which upgraded smol-toml from 1.7.0 to 1.8.0. This PR addresses the remaining warning in the same CI output by migrating mise's deprecated monorepo root setting before its scheduled removal.

Area

• cdk — infrastructure, handlers, constructs
• agent — Python runtime / Docker image
• cli — bgagent client
• docs — guides or design sources (docs/guides/, docs/design/)
• tooling — root mise.toml, scripts, CI workflows

Related

• GHSA-7w5x-hrqm-74c2, fixed by fix(deps): clear 7 osv advisories blocking the merge queue #870
• No tracking issue because this follows up on a blocking security CI failure

Changes

• Replace deprecated experimental_monorepo_root = true with monorepo_root = true
• Preserve monorepo task discovery while removing the warning scheduled to become an error after mise removes the old key

Verification

• MISE_EXPERIMENTAL=1 mise tasks --all — 75 tasks discovered, no deprecation warning
• mise run security:deps — No issues found, no deprecation warning
• mise run build — passed
  • CDK: 211 suites / 4,471 tests
  • CLI: 60 suites / 903 tests
  • Agent, docs, lint, and synth checks passed
• mise run install installed dependencies, then hit the documented local core.hooksPath conflict at the final prek install step

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.