[build] Replace tsc watch with esbuild watch mode for faster development

[vrubezhny]

• Split build scripts: esbuild.extension.cjs for extension, esbuild.webviews.cjs for webviews
• Add watch mode support to extension builds using esbuild.context()
• Update tasks.json with proper dependency chain (compile → watch-all → parallel watchers)
• Skip type-checking during watch mode (VS Code editor handles this)
• Add error handling to keep watchers running after build errors
• Reduces CPU usage from 80% to ~5-10% during development

Assisted-By: Claude Sonnet 4.5 noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.69%. Comparing base (da60441) to head (9929f87).
⚠️ Report is 1856 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5937       +/-   ##
===========================================
+ Coverage   32.37%   47.69%   +15.32%     
===========================================
  Files          85      106       +21     
  Lines        6505     9843     +3338     
  Branches     1349     2231      +882     
===========================================
+ Hits         2106     4695     +2589     
- Misses       4399     5145      +746     
- Partials        0        3        +3     

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