Document strict commit verification default in agent v4

[jamiemonserrate]

Description

Document that Buildkite agent v4 uses strict Git commit verification by default and that warn is the supported warning-only override.

Context

• A-1592: Consider git commit verification strict as default in v4
• Companion implementation: buildkite/agent#4145

Changes

• Add the new default and migration guidance to the agent v3-to-v4 upgrade guide.
• Update Git checkout and command-step documentation.
• Update the BUILDKITE_GIT_COMMIT_VERIFICATION environment-variable reference.
• Clarify that agent v3 does not verify commits unless configured.

Verification

• Markdownlint passes for the three changed Markdown pages.
• The environment-variable content validates against its schema.
• Vale reports no errors or warnings in the changed documentation.
• git diff --check passes.

Deployment

These documentation changes should be published alongside the agent v4 behavior change.

Rollback

Revert this commit if the agent behavior change is reverted.

Disclosures / Credits

OpenAI Codex prepared the documentation updates under Jamie Monserrate's direction.