recipes tool: Add tilde expansion and improve bundle path handling

Summary

The recipes tool's path resolution has two issues that cause confusing "Recipe file not found" errors:

Issue 1: Tilde (~) expansion not supported

When a path like ~/dev/my-recipes/recipe.yaml is passed to the recipes tool, it fails because tilde is not expanded.

Current code (amplifier-bundle-recipes/.../amplifier_module_tool_recipes/__init__.py:344-359):

def _resolve_path(self, path_str: str) -> Path | None:
    if path_str.startswith("@"):
        mention_resolver = self.coordinator.get_capability("mention_resolver")
        if mention_resolver is None:
            return None
        return mention_resolver.resolve(path_str)
    return Path(path_str)  # <-- No .expanduser() call

Fix: Add .expanduser() to the fallback path:

return Path(path_str).expanduser()

Issue 2: Bundle paths without @ prefix fail silently

When users pass bundle:path/to/recipe.yaml (without the @ prefix), it falls through to Path(path_str) which treats it as a literal path and fails.

This is confusing because:

• The bundle.md format uses bundle:path syntax (no @)
• The @ requirement is not immediately obvious

Options:

1. Document clearly that @ is required (done in amplifier-module-stories README)
2. Detect colon in path and provide a helpful error message suggesting the @ prefix
3. Auto-detect bundle paths (if path contains : and doesn't look like a Windows drive letter)

Reproduction

# Fails - tilde not expanded
amplifier tool invoke recipes operation=validate recipe_path="~/dev/my-bundle/recipes/test.yaml"

# Fails - missing @ prefix (confusing)
amplifier tool invoke recipes operation=validate recipe_path="my-bundle:recipes/test.yaml"

# Works
amplifier tool invoke recipes operation=validate recipe_path="@my-bundle:recipes/test.yaml"

Suggested Fix

Minimal change to _resolve_path():

def _resolve_path(self, path_str: str) -> Path | None:
    """Resolve a path string, handling @mention syntax."""
    if path_str.startswith("@"):
        mention_resolver = self.coordinator.get_capability("mention_resolver")
        if mention_resolver is None:
            return None
        return mention_resolver.resolve(path_str)
    
    # Expand ~ to home directory
    return Path(path_str).expanduser()

Impact

This affects any user who:

• Uses ~ in recipe paths (common pattern)
• Forgets the @ prefix on bundle paths (easy mistake)

Both result in the same unhelpful error: "Recipe file not found: [path]"

---

Discovered while using amplifier-module-stories recipes

Start at amplifier-bundle-recipes/.../amplifier_module_tool_recipes/init.py:344-359 and read _resolve_path(), then compare its handling of plain paths with @-prefixed bundle paths. Done means tilde-based recipe paths resolve correctly and the chosen behavior for bundle:path inputs is clear and covered by the tool's validation behavior.