Fix Bugbot gate contract and rollout docs

[thehoegh]

Summary

• fix the reusable Bugbot Gate workflow to evaluate the latest matching Bugbot check run for a SHA, avoiding stale rerun conclusions
• align timeout behavior by deriving job timeout from timeout_minutes with a buffer so script-level timeout failures are surfaced consistently
• sync gate.md and rollout handoff docs with the implemented contract (canonical names, workflow_run.head_sha checkout, downstream check names, and explicit conservative fork guidance)

Test plan

• review workflow logic in .github/workflows/bugbot-gate.yml to confirm latest-run polling and success-only completion behavior
• validate timeout expression and workflow syntax consistency (timeout-minutes: ${{ inputs.timeout_minutes + 2 }})
• cross-check gate.md and docs/bugbot-gate-rollout.md examples to ensure naming and ruleset guidance correspond to emitted checks
• manual smoke-check recommendation for pilot repo: rerun Bugbot on same SHA and verify the gate follows the newest run

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Note

Medium Risk
Changes CI gating behavior and required-check naming conventions, which can block PRs or waste minutes if misconfigured. Logic is straightforward but depends on correct check names/timeouts and GitHub Checks API behavior.

Overview
Introduces a shared reusable Bugbot Gate workflow (workflow_call) that polls GitHub Checks for the latest matching Bugbot check run on a provided SHA, failing on non-success conclusions or after a configurable timeout (with job timeout buffered via timeout_minutes + 2).

Adds rollout/hand-off documentation (docs/bugbot-gate-rollout.md and gate.md) that standardizes canonical check/workflow names, shows the thin per-repo caller pattern, and moves expensive CI behind a workflow_run trigger using github.event.workflow_run.head_sha (with explicit fork-safety guidance).

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[thehoegh]

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