docs: document the server-proxied GitHub device flow

[ProJedi1234]

The README's Credentials and Configuration sections still described the earlier exchange, where the CLI ran the device flow itself and handed this server a raw GitHub access token. Bring them in line with the two-route shape that replaced it: the API table, the two routes' own paragraphs, the closed provenance gap, the byte-identical refusal now covering both routes and its new shapes, the accepted device-code quota abuse, the replaced-exchange breaking change, and the "Enable Device Flow" deployment step.

PublishSkill/SKILL.md needs no change - it documents stele auth login as the user's step and says nothing about token mechanics, and PublishSkillTests (unaffected by this commit, since it reads PublishSkill.swift rather than README.md) is still green.

Tests: just build clean; just test: 424 tests in 38 suites, all passing — including PublishSkillTests, which is the suite this PR claims needs no change.

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