ACP mode: Skills in .agents/skills/ not loaded into system prompt

Describe the bug

In ACP mode (goose acp), Skills defined in .agents/skills/ are not loaded into the system prompt. Models answer from training data instead of triggering skills/tools.

The same skills work correctly in goose run with the same model and provider.

To Reproduce

1. Create a skill at .agents/skills/my-skill/SKILL.md
2. Run goose acp
3. Send initialize → session/new (with valid cwd) → session/prompt with a question matching the skill
4. Model answers from training data — no tool calls, no skill loading

Same skill, same model, same provider — different mode:

* Claude works around this by proactively calling Load() to discover skills, but the skill list in the system prompt is still empty.

Expected behavior

Skills in .agents/skills/ should be discovered during ACP session/new and included in the system prompt, same as goose run.

Suspected root cause

create_agent_for_session() builds ExtensionManager with session: None (hardcoded), even though session/new already received and stored a valid cwd. Because session is None, SummonClient cannot resolve the working directory → .agents/skills/ scan is skipped → skill list in the system prompt is empty → model has no skill to trigger.

session/new {cwd: "..."}
  → session_manager.create_session(cwd)          ✅ cwd saved
  → create_agent_for_session(session_id)
    → ExtensionManager::new(..., session: None)   ❌ not queried back
      → SummonClient: working_dir = None
        → skills scan skipped
          → system prompt skill list = empty

Environment

• OS & Arch: Windows 11 Enterprise x86_64
• Interface: CLI (goose acp)
• Version: v1.27.2
• Provider & Model: GitHub Copilot → GPT-4.1 / GPT-4o / GPT-5-mini / Claude Sonnet 4.6