feat(bundle): register 'anchors-amp-dev' (registration only, no default change)

[Salil Das (sadlilas)]

Summary

Registers anchors-amp-dev in WELL_KNOWN_BUNDLES — registration ONLY, no default changes. Users can explicitly select this bundle by name; existing defaults remain exactly as they are.

What this changes

Adds one entry to WELL_KNOWN_BUNDLES in amplifier_app_cli/lib/bundle_loader/discovery.py:

"anchors-amp-dev": {
    "remote": "git+https://github.com/microsoft/amplifier-foundation@main#subdirectory=bundles/anchors-amp-dev/bundle.md",
},

This makes the bundle selectable by name:

amplifier bundle use anchors-amp-dev

Key difference from anchors (#208)

#208 (the anchors PR) changed the CLI default to anchors. This PR does NOT change any defaults. We are adding anchors-amp-dev alongside amplifier-dev and anchors — all three coexist, the default remains anchors (as established by #208).

Testing

Regression tests pass (3/3):

tests/test_anchors_amp_dev_registration.py:
  ✓ Test anchors-amp-dev is registered in discovery
  ✓ Test default bundle is still 'anchors' (not changed by this PR)
  ✓ Test amplifier-dev coexists with anchors-amp-dev

Dependency

Requires the companion foundation PR #273 which adds bundles/anchors-amp-dev/bundle.md. That PR must merge first for this WELL_KNOWN_BUNDLES entry to resolve.

Context

anchors-amp-dev is a mechanical promotion of the experiment experiments/behavioral-anchor-amplifier-dev into a registered bundle. It was evaluated as a drop-in replacement for amplifier-dev across ~140 DTU eval runs and reached a drop-in verdict. This PR registers it in the CLI; the foundation PR publishes the bundle files.

[Salil Das (sadlilas)]

Cross-linked with foundation PR #273 (microsoft/amplifier-foundation#273) — that PR must merge first.