Remove is-interactive, inline the TTY check

[ryancbahan]

Summary

• Replace is-interactive package with inline logic: process.stdout.isTTY && TERM !== 'dumb' && !CI
• The package was 6 lines; the inline replacement is identical behavior
• Works cross-OS: isTTY is a Node.js built-in available on all platforms

Test plan

• 5 new tests covering all branches (TTY, non-TTY, dumb terminal, CI set, CI empty string)
• All 29 local.test.ts tests pass
• tsc --noEmit passes
• Lint passes

[ryancbahan]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Remove is-interactive, inline the TTY check #7198 👈 (View in Graphite)
• Remove http-proxy-node16, replace with lightweight inline proxy #7196
• Remove network-interfaces, use os.networkInterfaces() directly #7195
• Remove camelcase-keys, replace with inline utility #7194
• Remove ansi-colors, replace with lightweight inline utility #7193
• Remove tmp and @types/tmp dependencies #7192
• main

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[github-actions]

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