feat(templates): use {{ agent_display_name }} in all four base templates

[Copilot]

Summary

{{ agent_display_name }} was only referenced from 1es-base.yml and stage-base.yml. The standalone (base.yml) and job-target (job-base.yml) templates hard-coded displayName: "Agent" for the top-most user-facing label, so every pipeline rendered identically in the ADO UI regardless of the agent's name:.

• Templates: Replace the literal displayName: "Agent" on the Agent job in src/data/base.yml and src/data/job-base.yml with {{ agent_display_name }}, matching the existing stage-level usage in the 1ES and stage templates.
• Docs: Update docs/template-markers.md and site/src/content/docs/reference/template-markers.mdx to document the marker as used by all four base templates instead of a 1ES/stage-only subset.
• Snapshots: Regenerate the 26 dogfood tests/safe-outputs/*.lock.yml fixtures; the diffs also pick up incidental drift (compiler 0.30.1 → 0.30.2, Copilot CLI v1.0.47 → v1.0.48) from recompiling against the current binary.

Example diff for the standalone template:

  - job: Agent
-   displayName: "Agent"
+   displayName: {{ agent_display_name }}

…which now renders e.g. displayName: "Daily safe-output smoke: noop" instead of displayName: "Agent".

Test plan

• cargo test --release --test compiler_tests tricky — the YAML-escaping regression tests for tricky agent names (embedded " and :) still pass, confirming yaml_double_quoted handles the new substitution sites.
• cargo test --release — 103 passing; the 11 pre-existing --debug-pipeline failures reproduce on a clean main and are unrelated.
• Manually inspected git diff tests/safe-outputs/noop.lock.yml to confirm the only structural change is the Agent-job displayName flipping to the front-matter name.