feat(plh): flag plugin.json paths that shadow default folders (CA-PLH-015)

PLH now flags a plugin.json component-path key (commands/agents/outputStyles)
that replaces a default folder still present on disk — Claude Code stops
scanning that folder, so its contents are silently ignored (dead config).
Mirrors CC's /doctor & `claude plugin list` warning (v2.1.140+).

Field set pinned to the docs' "replaces" category only (Verifiseringsplikt,
code.claude.com/docs path-behavior-rules): skills is excluded (adds to the
default skills/ scan — both load) as are hooks/mcpServers/lspServers (own
merge rules); a custom path that addresses the default folder is not flagged.

Tests +5 (936->941). Scanner count unchanged (13). --json/--raw byte-stable.

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

tests/fixtures/plugin-shadow-folder/.claude-plugin/plugin.json

@@ -0,0 +1,9 @@
+{
+  "name": "shadow-folder-plugin",
+  "description": "Fixture: plugin.json manifest paths that shadow default folders (CA-PLH-015)",
+  "version": "1.0.0",
+  "commands": "./custom-cmds/",
+  "agents": ["./agents/", "./more-agents/"],
+  "skills": "./custom-skills/",
+  "outputStyles": "./styles/"
+}

tests/fixtures/plugin-shadow-folder/CLAUDE.md

@@ -0,0 +1,21 @@
+# Shadow Folder Plugin
+
+Fixture for CA-PLH-015 shadow-folder detection.
+
+## Commands
+
+| Command | Description |
+|---------|-------------|
+| `/shadow-folder-plugin:shadow-fixture-cmd` | A command |
+
+## Agents
+
+| Agent | Role | Model |
+|-------|------|-------|
+| shadow-fixture-agent | Test | sonnet |
+
+## Hooks
+
+| Event | Script | Purpose |
+|-------|--------|---------|
+| (none) | — | — |

tests/fixtures/plugin-shadow-folder/agents/real.md

@@ -0,0 +1,8 @@
+---
+name: shadow-fixture-agent
+description: An agent in the default agents/ folder
+model: sonnet
+tools: ["Read"]
+---
+
+# Shadow Fixture Agent

tests/fixtures/plugin-shadow-folder/commands/real.md

@@ -0,0 +1,8 @@
+---
+name: shadow-fixture-cmd
+description: A command that lives in the default commands/ folder
+allowed-tools: Read
+model: sonnet
+---
+
+# Shadow Fixture Command

tests/fixtures/plugin-shadow-folder/skills/some-skill/SKILL.md

@@ -0,0 +1,6 @@
+---
+name: shadow-fixture-skill
+description: A skill in the default skills/ folder
+---
+
+# Shadow Fixture Skill