[codex] integrate discovery interface contract extension

[bigsmartben]

What changed

Updates the bundled extensions/discovery snapshot to the source spec-kit-discovery interface-contract refactor.

• Replaces the old six-command discovery surface with the single /speckit.discovery command.
• Adds templates/interface-contract.md as the only persistent artifact template.
• Removes legacy feasibility, tech selection, codebase, PoC, and scenario-decision command/template files.
• Updates extensions/catalog.json and extensions/catalog.community.json discovery metadata to version 0.3.0, command count 1, and interface-contract validation tags.
• Updates extensions/discovery/CATALOG-SUBMISSION.md with source commit and validation evidence.

Source metadata

• Source repository: https://github.com/bigsmartben/spec-kit-discovery
• Source PR: [codex] refocus discovery on interface contracts spec-kit-discovery#4
• Source commit: 38f660c815f3ef95d80f143dac6c6411b12a9f04
• Download URL: https://github.com/bigsmartben/spec-kit-discovery/archive/38f660c815f3ef95d80f143dac6c6411b12a9f04.zip

Validation

• Confirmed extensions/discovery/commands contains only discovery.md.
• Confirmed extensions/discovery/templates contains only interface-contract.md.
• Confirmed extensions/discovery/extension.yml registers speckit.discovery with commands/discovery.md.
• Confirmed bundled and community catalog entries use version 0.3.0 and command count 1.
• Checked old discovery command/template residuals with rg; no matches.
• Ran git diff --check; passed.
• Ran custom manifest/catalog consistency check with uv run python; passed.
• Ran PYTHONPATH=src uv run pytest tests/contract/test_catalog_schema.py tests/test_extensions.py tests/integration/test_bundler_local_install.py: 339 passed, 3 skipped.
• Verified the source commit archive URL redirects to codeload and returns 200 OK.

Notes

This PR targets the integration fork only. It does not open or push anything to github/spec-kit.