fix(plh): downgrade cross-plugin command-name overlap to low ambiguity

Commands are namespaced (/name:command), so a command name shared by two
differently-named plugins keeps both reachable — it is ambiguity, not a hard
conflict. The check now mirrors COL's plugin-vs-plugin skill finding: severity
LOW (was HIGH), category plugin-hygiene, COL-shaped details.namespaces, and a
group-first shape (one finding per command name listing every namespace, not
pairwise). It keys on the declared namespace (was folder basename) and fires
only across 2+ distinct namespaces — when plugins share a declared name, the
namespace-collision finding (medium) is the right signal, so this stays silent.

Removes the inaccurate "only one wins" humanizer entry. Adds fixtures
(duplicate-command-name; a shared command in duplicate-plugin-name's colliding
namespace) and 4 tests. Suite 932->936. self-audit A 97 / A 100, scanners 13.

tests/fixtures/duplicate-command-name/one/.claude-plugin/plugin.json

@@ -0,0 +1 @@
+{ "name": "plugin-one", "description": "Declares namespace plugin-one", "version": "1.0.0" }

tests/fixtures/duplicate-command-name/one/CLAUDE.md

@@ -0,0 +1,12 @@
+# Plugin
+
+## Commands
+| Command | Description |
+|---------|-------------|
+| `/shared-cmd` | shared |
+
+## Agents
+(none)
+
+## Hooks
+(none)

tests/fixtures/duplicate-command-name/one/commands/shared.md

@@ -0,0 +1,7 @@
+---
+name: shared-cmd
+description: A command whose name is shared across two differently-named plugins
+model: sonnet
+allowed-tools: Read
+---
+Do the thing.

tests/fixtures/duplicate-command-name/two/.claude-plugin/plugin.json

@@ -0,0 +1 @@
+{ "name": "plugin-two", "description": "Declares namespace plugin-two", "version": "1.0.0" }

tests/fixtures/duplicate-command-name/two/CLAUDE.md

@@ -0,0 +1,12 @@
+# Plugin
+
+## Commands
+| Command | Description |
+|---------|-------------|
+| `/shared-cmd` | shared |
+
+## Agents
+(none)
+
+## Hooks
+(none)

tests/fixtures/duplicate-command-name/two/commands/shared.md

@@ -0,0 +1,7 @@
+---
+name: shared-cmd
+description: A command whose name is shared across two differently-named plugins
+model: sonnet
+allowed-tools: Read
+---
+Do the thing.