docs: skill month/week date alignment and OAuth access_denied notes

[jdwit]

Two doc gaps in skills/ytstudio/SKILL.md.

• analytics query: caveat that -s and -e must both be the first of a month for -d month/-d week (the API rejects other days and does not snap), that the range is inclusive on both ends, plus a working 12-month example.
• OAuth setup: short access_denied troubleshooting note (use the channel-owning account, add it as a Test user while the app is in Testing, click through the unverified-app warning, approve all read-only scopes).

Docs-only. references/reference.md is generated from the CLI and the CLI surface did not change, so it was not regenerated. tests/test_skill_reference.py and the full suite pass; ruff clean.

Closes #40

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.90%. Comparing base (941f1b6) to head (387e2e9).

Additional details and impacted files

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##             main      #42   +/-   ##
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  Coverage   83.90%   83.90%           
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  Files          15       15           
  Lines        2460     2460           
  Branches      356      356           
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  Hits         2064     2064           
  Misses        264      264           
  Partials      132      132           

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