simplify init and add a login prompt

[tulioz]

drops the who is this for question, offers a sign in when you have no credentials, and prints the commit plus teammate steps at the end

[aviator-app]

Current Aviator status

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This PR was merged using Aviator (commit d2ee651).

Stack

• 👉 #23 simplify init and add a login prompt (this PR)
• master

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[aviator-app]

✅ FlexReview Status

Common Owner: aviator-co/engineering (expert-load-balance assignment)
Owner and Assignment:

• ✅ aviator-co/engineering (expert-load-balance assignment)
  • Slack: #fr-eng-notifications
  • Assigned: jainankit
  Owned Files

  • 🔒 README.md
  • 🔒 cmd/aviator/hooks.go
  • 🔒 cmd/aviator/init.go
  • 🔒 cmd/aviator/login.go
  • 🔒 internal/adapter/agents.go
  • 🔒 internal/api/credentials.go

Review SLO: 7 business hours if PR size is <= 200 LOC for the first response.
❕ This PR modifies 209 lines, which is larger than the Review SLO threshold.