fix(opencode-skill-loader): align getSkillByName with short-name matching (fixes #4183)

PR #4146 fixed the async resolvers (resolveSkillContentAsync, resolveMultipleSkillsAsync) to support unambiguous short-name skill lookups by routing through matchSkillByName. But the public getSkillByName API (exported from src/features/opencode-skill-loader/) was left on the old skills.find(s => s.name === name) lookup, so any external caller asking for 'systematic-debugging' on a 'superpowers/systematic-debugging' skill silently gets undefined.

Wire getSkillByName through the same matchSkillByName helper so the public API matches the async resolution semantics:
- Exact full-name lookup (case-insensitive) is preferred.
- Unambiguous short-name lookup falls back to the unique namespaced match.
- Ambiguous short-name (>=2 namespaces share the same short tail) returns undefined, mirroring matchSkillByName.

Regression tests added to src/features/opencode-skill-loader/loader.test.ts under a new 'getSkillByName' describe block:
- exact full-name returns the skill
- namespaced skill resolves by its unique short name
- ambiguous short name returns undefined

Verification: 3/3 new cases pass, 129/129 full src/features/opencode-skill-loader tests pass (was 126/126 pre-fix), bun run typecheck clean.

src/features/opencode-skill-loader/loader.test.ts

@@ -703,4 +703,90 @@ Skill body.
       }
     })
   })
+
+  describe("getSkillByName", () => {
+    it("#given a discoverable skill #when getSkillByName is called with the exact full name #then it returns the skill", async () => {
+      // given - a skill with a plain (non-namespaced) name
+      const skillContent = `---
+name: my-exact-skill
+description: A skill resolvable by exact name
+---
+Body.
+`
+      createTestSkill("my-exact-skill", skillContent)
+
+      // when
+      const { getSkillByName } = await import("./loader")
+      const originalCwd = process.cwd()
+      process.chdir(TEST_DIR)
+
+      try {
+        const skill = await getSkillByName("my-exact-skill", { includeClaudeCodePaths: false })
+
+        // then
+        expect(skill).toBeDefined()
+        expect(skill?.name).toBe("my-exact-skill")
+      } finally {
+        process.chdir(originalCwd)
+      }
+    })
+
+    it("#given a namespaced skill #when getSkillByName is called with its unique short name #then it returns the skill", async () => {
+      // given - a namespaced skill that is the unique short-name match
+      const skillContent = `---
+name: superpowers/systematic-debugging
+description: Namespaced skill the agent should be able to load by short name
+---
+Body.
+`
+      createTestSkill("systematic-debugging", skillContent)
+
+      // when
+      const { getSkillByName } = await import("./loader")
+      const originalCwd = process.cwd()
+      process.chdir(TEST_DIR)
+
+      try {
+        const skill = await getSkillByName("systematic-debugging", { includeClaudeCodePaths: false })
+
+        // then - the short-name lookup must succeed, mirroring matchSkillByName semantics
+        expect(skill).toBeDefined()
+        expect(skill?.name).toBe("superpowers/systematic-debugging")
+      } finally {
+        process.chdir(originalCwd)
+      }
+    })
+
+    it("#given two namespaced skills sharing a short name #when getSkillByName is called with that short name #then it returns undefined (ambiguous)", async () => {
+      // given - two skills under different namespaces with the same short name
+      const skillA = `---
+name: alpha/duplicated
+description: Skill A
+---
+Body A.
+`
+      const skillB = `---
+name: beta/duplicated
+description: Skill B
+---
+Body B.
+`
+      createTestSkill("alpha-duplicated", skillA)
+      createTestSkill("beta-duplicated", skillB)
+
+      // when
+      const { getSkillByName } = await import("./loader")
+      const originalCwd = process.cwd()
+      process.chdir(TEST_DIR)
+
+      try {
+        const skill = await getSkillByName("duplicated", { includeClaudeCodePaths: false })
+
+        // then - ambiguous short-name match must NOT resolve, matching matchSkillByName behavior
+        expect(skill).toBeUndefined()
+      } finally {
+        process.chdir(originalCwd)
+      }
+    })
+  })
 })

src/features/opencode-skill-loader/loader.ts

@@ -8,6 +8,7 @@ import {
   findProjectClaudeSkillDirs,
   findProjectOpencodeSkillDirs,
 } from "../../shared/project-discovery-dirs"
+import { matchSkillByName } from "../../tools/skill/skill-matcher"
 import type { CommandDefinition } from "../claude-code-command-loader/types"
 import type { LoadedSkill } from "./types"
 import { skillsToCommandDefinitionRecord } from "./skill-definition-record"
@@ -122,7 +123,7 @@ export async function discoverSkills(options: DiscoverSkillsOptions = {}): Promi
 
 export async function getSkillByName(name: string, options: DiscoverSkillsOptions = {}): Promise<LoadedSkill | undefined> {
   const skills = await discoverSkills(options)
-  return skills.find(s => s.name === name)
+  return matchSkillByName(skills, name)
 }
 
 export async function discoverUserClaudeSkills(): Promise<LoadedSkill[]> {