refactor: copilot brand/term updates in UI & templates

[yuqizhou77]

• Copilot for Microsoft 365 -> Microsoft 365 Copilot
• Copilot extension -> Copilot agent
• declarative copilot -> declarative agent
• custom engine copilot -> custom engine agent

Changes included:

• UI
• Template file name
• Template readme (including screenshot)
• build intelligent app walkthrough
• CLI option id, id: declarative-agent

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.61%. Comparing base (56107d2) to head (8d0aa86).
Report is 7 commits behind head on release/5.10.0.

Additional details and impacted files

@@               Coverage Diff               @@
##           release/5.10.0   #12386   +/-   ##
===============================================
  Coverage           94.61%   94.61%           
===============================================
  Files                 582      582           
  Lines               34494    34494           
  Branches             6788     6708   -80     
===============================================
  Hits                32638    32638           
  Misses               1582     1582           
  Partials              274      274           

Files with missing lines	Coverage Δ
...-core/src/component/generator/apiSpec/generator.ts	97.27% <100.00%> (ø)
.../component/generator/copilotExtension/generator.ts	100.00% <100.00%> (ø)
packages/fx-core/src/question/constants.ts	97.48% <ø> (ø)
...-core/src/question/options/CreateProjectOptions.ts	100.00% <ø> (ø)