* refactor(installer): discover skills by SKILL.md instead of manifest YAML
Switch skill discovery gate from requiring bmad-skill-manifest.yaml with
type: skill to detecting any directory with a valid SKILL.md (frontmatter
name + description, name matches directory name). Delete 34 stub manifests
that carried no data beyond type: skill. Agent manifests (9) are retained
for persona metadata consumed by agent-manifest.csv.
* refactor(installer): remove dead task/tool/workflow manifest code
The remove-skill-manifest-yaml branch deleted the scanners that
discover tasks, tools, and workflows but left behind the code that
writes their manifest CSVs. Remove collectTasks/Tools/Workflows,
writeTaskManifest/ToolManifest/WorkflowManifest, their helpers, and
the now-unreachable getPreservedCsvRows/upgradeRowToSchema methods.
Update installer pre-registration and test assertions accordingly.
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