Use Imagile Bot token for automated triage pull requests

[kolatts]

Problem

Pull requests created by the Claude triage workflow use the repository GITHUB_TOKEN. GitHub places workflows triggered by those PRs into an approval-required state, preventing fully automatic CI and review execution.

Proposed change

• Authenticate branch pushes and PR creation as the installed Imagile Bot GitHub App instead of github-actions[bot].
• Generate a short-lived installation token with actions/create-github-app-token.
• Store the App ID as a repository Actions variable and its private key as an encrypted Actions secret.
• Keep the built-in GITHUB_TOKEN permissions least-privileged.
• Verify the workflow configuration without exposing credentials.

Acceptance criteria

• Triage-created branches and PRs are attributed to Imagile Bot.
• PR workflows start without the special approval required for GITHUB_TOKEN-created PRs.
• No private-key material is committed or logged.
• Existing issue triage, branch push, PR creation, and issue-linking behavior remains intact.