feat(extensions): consolidated selection menu and community extensions framework

[ClintEastman02]

Summary

• Consolidated selection menu: Replaces per-extension opt-in prompts with a single table during Requirements Analysis — scales to N extensions with one question
• Community extensions index: Adds community-extensions-index.yaml at repo root establishing the decentralized extension contract (Proposal C from RFC: Community Extensions Framework #225)
• Extensions README: Documents loader behavior, available extensions, and community extension install process

What Changed

File	Change
inception/requirements-analysis.md	Step 5.1: per-extension opt-in → consolidated selection menu
extensions/README.md	New — documents loader, lists available extensions, install instructions
community-extensions-index.yaml	New — empty index template with schema for community extensions
README.md	Updated Extensions section for consolidated menu, added Community Extensions subsection
docs/WORKING-WITH-AIDLC.md	Updated advanced tip to describe menu instead of per-extension opt-in

Alignment with #225

#225 Consensus	This PR
Scott: "Proposal B + Proposal C"	Official extensions in extensions/, community via index
Jeff: "SHA-256 integrity pinning"	Index schema includes sha256 per file, mandatory at reviewed maturity
Jeff: "Maturity lifecycle"	new → incubating → reviewed → adopted in index schema
Jeff: "No extension dependencies for v1"	Not implemented — kept simple
Unified loader	Same *.opt-in.md scanning handles both official and installed community extensions

Relationship to NIST 800-53

This PR establishes the framework only. The NIST 800-53 compliance extension (the first community/official extension using this framework) will follow as a separate PR.

Test plan

• Run test scenario with extensions — verify consolidated selection menu appears during Requirements Analysis
• Verify selection of "none" skips all extensions
• Verify selection of specific numbers enables only those extensions
• Verify aidlc-state.md records extension enablement status
• Verify community-extensions-index.yaml schema is valid YAML

Full test results posted in PR comment.

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[ClintEastman02]

Test Results ✅

All test scenarios passed:

Test	Result
Run with extensions — consolidated selection menu appears during Requirements Analysis	✅ Pass
Selection of "none" skips all extensions	✅ Pass
Selection of specific numbers enables only those extensions	✅ Pass
aidlc-state.md records extension enablement status	✅ Pass
community-extensions-index.yaml schema is valid YAML	✅ Pass
Existing security-baseline and property-based-testing extensions appear in menu	✅ Pass
Extensions loader ignores README.md (only scans subdirectories)	✅ Pass

All tests executed manually against the consolidated selection menu flow. No regressions observed in core workflow behavior.