Unify the canonical golden snippet across README, AGENTS, llms.txt

[fabioknoedt]

Same six lines, byte-identical, in every entrypoint that matters:

import YCFirstTime

YCFirstTime.shared.executeOnce({
    // your one-time code, e.g. showOnboarding()
}, forKey: "onboarding.v1")

Three changes:

• README: hoist the snippet to right under the metadata block, before
  the "Copy-paste integration" heading. It's now the first code a
  reader (or LLM scraping the top of the README) sees.
• README: change the body comment in the in-flow occurrence from
  "// Onboarding that runs exactly once per install." +
  showOnboarding() to the generic "your one-time code" comment, so
  both README occurrences match each other.
• AGENTS.md: same comment change in its "Run once per install" block.

llms.txt was already in this exact form. Total: four occurrences
across three files, all identical.

The snippet uses the actual public API. The earlier sketch
(YCFirstTime.executeOnce(key:) { ... }) wouldn't compile —
shared is required, and the trailing-block / forKey: shape
matches both Obj-C selector compatibility and the existing
documentation.

https://claude.ai/code/session_01TQi3WNVCqQ9QFufs5XogjK

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (1f6d791) to head (07009f1).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #37   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           83        83           
=========================================
  Hits            83        83           

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