docs(readme): add TypeScript version requirement under Installation

[imjuni]

Summary

• Add a callout block under the Installation section explaining that ctix 2.8.0 requires TypeScript >=5.0.0 <6.0.0
• TypeScript 6.x introduces ESM-first defaults with breaking changes to moduleResolution and baseUrl not yet supported by ctix
• Include an explicit install command so users can pin a compatible TypeScript version

Test plan

• Verify the callout renders correctly on GitHub and npm README
• Confirm the TypeScript version range matches peerDependencies in package.json

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7230fe2) to head (3ff30f8).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #196   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           94        94           
  Lines         2510      2510           
  Branches       526       526           
=========================================
  Hits          2510      2510           

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