docs(14): document sharing architecture trade-off

[FSM1]

Summary

• Documents the architectural trade-off between server-side share records (current) vs metadata-embedded ACLs (considered alternative)
• Records revocation risk assessment — practical risk is low in both approaches
• Preserves the metadata-embedded approach as a viable future option for hiding the social graph

Test plan

• No code changes — docs only review

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Added architectural trade-off analysis documenting implementation approaches and considerations for the sharing feature.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Walkthrough

Adds an Architecture Trade-Off section to the planning documentation for Phase 14. The section compares server-side share records versus metadata-embedded ACLs approaches, documenting consequences, advantages, disadvantages, and revocation risk considerations, with a decision favoring the server-side approach.

Changes

Cohort / File(s)	Summary
Planning Documentation .planning/phases/14-user-to-user-sharing/14-CONTEXT.md	Adds Architecture Trade-Off section detailing server-side share records vs. metadata-embedded ACLs comparison with analysis of consequences, advantages, disadvantages, and revocation risks.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: documenting an architectural trade-off for the sharing feature. It is concise, specific, and directly related to the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch docs/phase-14-architecture-tradeoff

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