🌱 Replace local governance workflows with llm-d-infra callers

[clubanderson]

Summary

• Replace all 9 local governance workflows with thin callers to llm-d/llm-d-infra reusable workflows
• Covers: Prow commands, automerge, remove-lgtm, stale/unstale, signed-commits, typos, md-link-check, non-main-gatekeeper
• Net reduction of ~163 lines of duplicated workflow logic

This is a companion to PR #613 (pre-commit config). That PR adds pre-commit; this PR migrates governance workflows.

Test plan

• Verify Prow commands (/lgtm, /approve) still work
• Confirm stale/unstale behavior is unchanged
• Verify signed-commits check still runs

Depends on: llm-d/llm-d-infra#2 (already merged)

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[clubanderson]

@nilig can you ptal - centralizing workflows to make this easier to maintain

[clubanderson]

PTAL @elevran @nirrozenbaum 👋

This PR is part of an org-wide effort to unify and centralize governance workflows (prow commands, auto-merge, stale issue management, signed-commit checks, typo checking, markdown link validation, etc.) into a single shared repo: llm-d/llm-d-infra.

Why? Currently these identical workflows are copy-pasted across 8+ repos, making maintenance painful. By centralizing them as reusable workflows, we get:

• Single source of truth — fix once, all repos benefit
• Easier maintenance — version updates, bug fixes in one place
• Consistency — every repo gets the same governance behavior

This PR replaces the local workflow copies with thin callers that reference the shared workflows. No behavior changes — just consolidation.

[clubanderson]

Consolidated into #616