Split Evergreen PR trigger from agent workflow

[mrjf]

Summary

• Removes direct pull_request activation from the agentic Evergreen workflow.
• Adds workflow_call inputs to .github/workflows/evergreen.md and recompiles .github/workflows/evergreen.lock.yml.
• Adds a plain .github/workflows/evergreen-trigger.yml using pull_request_target to gate labeled PR events before calling the reusable Evergreen lockfile.
• Passes pr_number and head_sha into the callable workflow; Evergreen re-fetches the PR and no-ops if the head SHA has changed.

Why

The prior pull_request trigger could hit gh-aw confused-deputy protection on bot-authored PRs: the run appeared successful, but pre-activation skipped the agent. This keeps fast PR-event reaction without handing raw PR-event context directly to the agentic workflow.

Verification

• gh aw compile succeeded; existing unrelated Autoloop/CI Doctor warnings were emitted.
• gh aw compile evergreen succeeded with 0 errors and 0 warnings.
• actionlint .github/workflows/evergreen-trigger.yml
• git diff --check
• apm compile attempted; repo is not an APM project because no apm.yml exists.

Note: I scoped the commit to Evergreen files and did not include unrelated lockfile churn from the full gh aw compile run.