docs: update frontend design guidelines

[potter-sun]

Summary

• expand docs/canon/frontend-design.md into a Console Web design baseline that covers product positioning, page models, CQRS/Observation/ReadModel UI semantics, frontend dependency boundaries, state display standards, responsive priorities, and validation requirements
• add docs/discussions/2026-05-11-frontend-architecture-discussion.md as a PM-oriented team discussion doc for Console Web frontend design, navigation, page responsibilities, data contracts, shared components, priorities, and success metrics

Why

The Console Web design guidance needs to describe more than visual style. Aevatar's frontend has to help users understand and operate AI Teams without leaking backend internals or faking consistency. This PR aligns frontend guidance with Team / Member / Service / Run / Governance workflows and makes the frontend contract around commands, observations, and read models explicit.

Impact

• Documentation-only change
• Gives frontend contributors a clearer design baseline for Console Web, CLI playground, and demo surfaces
• Gives product/frontend/backend contributors a concrete discussion artifact for deciding navigation, workbench boundaries, shared observation components, and data-source ownership

Validation

• bash tools/docs/lint.sh
• git diff --check
• git diff --cached --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.02%. Comparing base (36f96f7) to head (4d8a10b).

@@           Coverage Diff           @@
##              dev     #610   +/-   ##
=======================================
  Coverage   72.01%   72.02%           
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  Files        1255     1255           
  Lines       90723    90723           
  Branches    11877    11877           
=======================================
+ Hits        65338    65343    +5     
+ Misses      20703    20699    -4     
+ Partials     4682     4681    -1     

Flag	Coverage Δ
ci	72.02% <ø> (+<0.01%)	⬆️

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see 1 file with indirect coverage changes

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