Fix skill frontmatter parsing on the current course

[Bill-Billion]

Summary

• require standalone YAML frontmatter delimiters in S07 and the integrated S15 harness
• fall back safely for malformed, empty, or non-string metadata and ignore manifests that resolve outside the skills directory
• add regression coverage for both runtimes and refresh generated course data

Context

PR #434 targets the course layout before #512. This PR applies the same issue to the current structure, including the integrated harness inherited by S16.

Fixes #430

Supersedes #434

Verification

• pytest -q tests/test_skill_loading.py tests/test_agent_teams_runtime.py tests/test_workflow_goal_lessons.py tests/test_web_scenarios.py::test_generated_s16_metadata_extends_s15_without_registry_false_positives -> 79 passed, 65 subtests passed
• pytest -q -k "not test_s16_scenario_matches_the_deterministic_runtime" -> 158 passed, 1 deselected, 84 subtests passed
• npm --prefix web run extract -> generated data is idempotent
• git diff --check

The excluded S16 scenario test also fails on the unchanged main commit because concurrent progress events can complete in a different order than the fixture.

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Status	Name	Latest Commit	Updated (UTC)
❌ Deployment failed View logs	learn-claude-code	acdeacc	Aug 14 2026, 03:17 PM