Refactor launcher path resolution in index.js

[gugu8intel-i9]

Micro-optimizes the entrypoint:

• 'use strict'
• Hoisted path constants
• Single existsSync + ternary require
• Identical behavior and fallback logic preserved
• Tested and ready to push

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[codebuff-team]

Thanks for the PR, but there's no substantive change here to port. The diff:

• Renames packagedLauncherPath/sourceLauncherPath to packaged/source — purely cosmetic.
• Renames error to err in the catch handler — cosmetic, and this file already used a descriptive name deliberately.
• Adds 'use strict' — harmless but meaningless in CommonJS given Node's default semantics here don't change behavior; not a real fix.
• Removes the comment explaining why the packaged path must be checked first ("Published packages must not let an unrelated sibling path shadow their bundled launcher..."). That comment documents real reasoning about load-order safety for published vs. source checkouts. Deleting it is a net loss of information for future maintainers, not an improvement.

The PR description claims this is a 'micro-optimization,' but there is no optimization: it's the same fs.existsSync + ternary require logic, just with shorter variable names. Nothing here changes runtime behavior, fixes a bug, or improves clarity — if anything it slightly hurts clarity by dropping the rationale comment.

Since the private repo only ports substantive changes, this doesn't clear that bar. Happy to look at a follow-up if you find an actual behavioral issue in this launcher resolution logic (e.g., an edge case where both paths exist, or where require.main checks fail in bundled context).