docs: the last three "method" strings say skill

[devin-ai-integration]

Summary

#62 renamed methods → skills; three strings survived it. Two are gate failure messages a contributor only ever sees when something is already broken (unrouted methods are dead code, dead method: …), and one is reader-facing: the | Method highlights | column in docs/skills-reference.md's phase table.

Strings only — no logic, no counts, no version bump. npm run check green, 121 tests.

"Method" still stands where it describes what a skill is — "each skill is a method, not advice: thinking, artifact, checkpoint" — which is the line that stops a reader expecting a tip sheet.

Link to Devin session: https://app.devin.ai/sessions/f135381c4682413bae73dff38eb6d1a3
Open in Devin Desktop: https://app.devin.ai/desktop/session/f135381c4682413bae73dff38eb6d1a3?variant=devin
Requested by: @suboss87

[devin-ai-integration]

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