fix: clarify Studio member creation flow

[potter-sun]

Summary

• Clarify the Studio Create member modal so only Workflow uses the immediate member creation form.
• Keep Script and GAgent visible as member type choices, but route them to their Build workspaces instead of asking for a fake member name.
• Add the Studio Script member flow implementation plan under docs/design/.

Issue Links

Closes #441
Related to #442

Impact

Workflow creation remains unchanged. Script and GAgent now behave as honest builder entry points until their direct member creation APIs land.

Validation

npm --prefix apps/aevatar-console-web run test:ui -- --runTestsByPath src/pages/studio/index.test.tsx --runInBand
npm --prefix apps/aevatar-console-web run tsc

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.90%. Comparing base (7c89f22) to head (0e96fcc).

@@            Coverage Diff             @@
##              dev     #443      +/-   ##
==========================================
- Coverage   70.90%   70.90%   -0.01%     
==========================================
  Files        1209     1209              
  Lines       87045    87045              
  Branches    11411    11411              
==========================================
- Hits        61722    61720       -2     
- Misses      20876    20878       +2     
  Partials     4447     4447              

Flag	Coverage Δ
ci	70.90% <ø> (-0.01%)	⬇️

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see 1 file with indirect coverage changes

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