fix: propagate tags from skill frontmatter to SkillResource

[clawtom]

Summary

Fixes #3402.

SkillsProvider (and SkillProvider) were constructing SkillResource objects without populating the tags field, even when tags were declared in the skill's YAML frontmatter. This prevented tag-based filtering from working for skill resources.

Root cause

In skill_provider.py, both the list_resources() and get_resource() paths constructed SkillResource with an empty/default tags set:

SkillResource(
    uri=AnyUrl(f"skill://{skill.name}/{self._main_file_name}"),
    name=f"{skill.name}/{self._main_file_name}",
    description=skill.description,
    mime_type="text/markdown",
    # tags were never passed — defaults to set()
    skill_info=skill,
    is_manifest=False,
)

The tags data is available in skill.frontmatter (parsed from YAML frontmatter), but was never forwarded to SkillResource.

Fix

Pass tags=set(skill.frontmatter.get("tags", [])) in both construction sites (list and get paths). The manifest resources are intentionally left without tags, as they represent directory listings rather than the skill content itself.

Testing

To reproduce before fix:

---
description: My skill
tags: [python, testing]
---

resources = await provider.list_resources()
# resources[0].tags == set()  # wrong — should be {"python", "testing"}

After fix, resources[0].tags == {"python", "testing"}.