[cd] Gate release bot credentials behind GitHub environments (#100121)
Moves access to Next.js bot behind GitHub environments to ensure the bot
only runs code that has been reviewed by at least two people with write
access.
New GitHub environments:
- `bot-autonomous`
- `bot-approved`
## Testing
- Verified all 6 remaining key consumers on canary are covered (`git
grep RELEASE_GITHUB_APP_PRIVATE_KEY` only finds the environment-backed
mint steps).
- After settings land: dispatch canary with `force=false` right after a
release (exits early but exercises env + mint), and dispatch stable with
`dryRun=true` to watch the `bot-approved` approval gate trigger with no
side effects.
- The nightly canary is the real soak test for `bot-autonomous`.
Backports: #100123 (`releases/lts/active`), #100124
(`releases/lts/maintenance`)
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Co-authored-by: Claude Code <noreply@anthropic.com>