fix(plh): align required-frontmatter with CC docs (drop optional model/tools/name)

plugin-health-scanner required `model`+`allowed-tools` on commands and `model`+`tools`
on agents, plus `name` on commands. Per primary docs these are OPTIONAL:
- Commands/skills (code.claude.com/docs slash-commands): "All fields are optional. Only
  `description` is recommended." `name` defaults to the directory name.
- Subagents (code.claude.com/docs sub-agents): "Only `name` and `description` are required";
  `model` defaults to `inherit`, `tools` inherits all.

REQUIRED_COMMAND_FRONTMATTER -> [description]; REQUIRED_AGENT_FRONTMATTER -> [name, description].
This was over-reporting: every command without an explicit `model` got a spurious medium
finding (10 on the okr plugin alone). Found via the okr pilot review. Suite 949/949, self-audit
A/A, scanner count 13 (no new scanner).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ter3E2JSi1Khgmuf2kady8

tests/fixtures/broken-plugin/agents/bad-agent.md

@@ -1,8 +1,8 @@
 ---
 name: bad-agent
-description: Missing model and tools
 ---
 
 # Bad Agent
 
-No model or tools in frontmatter.
+Has a `name` but no `description`. Per CC sub-agents docs, `name` and `description` are
+required for subagents; `model` and `tools` are optional (inherit / all-tools by default).