fix(bmm): tighten bmad-investigate per review and quality findings

Maintainer review (PR #2364):
- shorten frontmatter description
- add customize.toml for case-file template, output subdir, filename,
  persistent_facts, prepend/append steps, on_complete
- subagent delegation discipline for context-heavy investigation steps
- replace ambiguous "Halt" closer at each outcome with self-documenting
  "Pause for user with the recap above; wait for direction."

Workflow-builder quality findings:
- move case-file-template.md to references/ (path standards)
- align activation with sibling 4-implementation skills (customize.toml
  resolution, persistent_facts, greet)
- operationalize promised flows: existing-case resume (Outcome 0),
  evidence-light branch (Outcome 1), refutation pass (Outcome 3),
  trivial-fix hand-off (Outcome 4)
- define High/Medium/Low confidence in Outcome 5
- name concrete next-step skills at Outcome 5 (bmad-quick-dev,
  bmad-correct-course, bmad-create-story, bmad-code-review)
- add Hand-off Brief and Side Findings to case-file template