fix(gate): require identity for human-authenticated agent PRs

[ArchonVII]

Problem

Agents can use a human-authenticated gh session, so GitHub sees the PR as the human account instead of a bot. In that state, bot-only PR body auto-injection and agent identity assumptions do not apply.

A malformed non-draft PR can also start expensive language CI even when the cheap PR contract already makes the required gate impossible to pass.

Acceptance Criteria

• Reusable PR contract logic can require configured human-authenticated agent logins to use an agent-identifying branch/body contract.
• repo-required-gate does not run expensive language/snapshot/dependency validation when a required PR contract fails first.
• PR body auto-inject can be configured for human-authenticated agent logins, not just bot accounts.
• Tests cover the agent-auth identity rule and preserve normal human/default behavior.
• Example callers document the new configuration knobs.