feat(core,engine,compound-engineering): make CE sub-agent sessions load their skills

Closes the U2/U5 skill-discovery carry-forward so the plugin's interactive ce-*
sessions actually load the stage's bundled skill in a live agent (not just in
scripted-fake tests).

Root cause: createFnAgent built its DefaultResourceLoader without forwarding any
skill-discovery path, and the interactive seam options couldn't carry one. The
loader's skillsOverride only *filters* skills already discovered from cwd's
standard roots, so the plugin-local .fusion-ce-skills/<id>/SKILL.md was never
discoverable.

Fix (end-to-end):
- AgentOptions.additionalSkillPaths forwarded into DefaultResourceLoader
- CreateInteractiveAiSessionOptions gains requestedSkillNames + additionalSkillPaths
- the interactive engine adapter forwards them to createFnAgent (skills +
  additionalSkillPaths)
- the orchestrator runs the session with cwd on the real project root and hands
  it [stage.skillId] + the install root

Proven: a real DefaultResourceLoader with additionalSkillPaths discovers ce-plan
and filters out ce-work; the orchestrator passes the right id/path/cwd. Plugin 96,
engine 136, core 99 tests green.

packages/engine/src/index.ts

@@ -156,6 +156,11 @@ const _createInteractiveAiSessionAdapter: CreateInteractiveAiSessionFactory = (
         tools: opts.tools,
         defaultProvider: opts.defaultProvider,
         defaultModelId: opts.defaultModelId,
+        // Forward skill selection so a plugin can load a specific bundled skill.
+        // `skills` (convenience) auto-builds a SkillSelectionContext; the extra
+        // discovery dirs make those skills actually visible to the loader.
+        ...(opts.requestedSkillNames?.length ? { skills: opts.requestedSkillNames } : {}),
+        ...(opts.additionalSkillPaths?.length ? { additionalSkillPaths: opts.additionalSkillPaths } : {}),
       }),
     options,
   );