docs: design relationship-based control (#87)

[e7217]

Summary

• add ADR 0003 for relationship-based control and conditional triggers
• define control subjects, message envelopes, safety modes, audit behavior, and follow-up implementation slices
• link ADR 0003 from README roadmap and ADR index

Test

• go test ./...

[Copilot]

Pull request overview

Adds an Architecture Decision Record (ADR) documenting the planned design for relationship-based control and conditional triggers, and links it from the project roadmap and ADR index.

Changes:

• Add ADR 0003 describing the control plane (platform.control.*), message envelopes, safety modes, propagation policy, audit behavior, and implementation slices.
• Link ADR 0003 from the main README roadmap.
• Add ADR 0003 to the ADR index.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
README.md	Updates the roadmap to link Phase 2 items to ADR 0003.
docs/adr/README.md	Adds ADR 0003 to the ADR list.
docs/adr/0003-relationship-based-control.md	New ADR describing relationship-based control + conditional triggers design and validation criteria.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.61%. Comparing base (23ebf93) to head (d474135).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #93       +/-   ##
===========================================
+ Coverage   46.76%   63.61%   +16.85%     
===========================================
  Files           5       15       +10     
  Lines         355     2053     +1698     
===========================================
+ Hits          166     1306     +1140     
- Misses        168      537      +369     
- Partials       21      210      +189     

see 2 files with indirect coverage changes

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