feat(common): surface skill file path in useSkill tool result

[wsxiaoys]

Summary

• Surfaces the skill's file path in the useSkill tool result prompt text.
• Encourages the agent/model to resolve relative resource paths (such as scripts/, references/, assets/) relative to the skill's directory.
• Adds comprehensive test coverage for createUseSkillResult to verify path inclusion and correct fallback behavior.

Test plan

• Run unit tests: bun run test -- src/base/prompts/__tests__/skill.test.ts (verified passing).
• Run typechecking and lint checks.

Context / Background

Here is the error/issue showing why this is needed (before this change, the agent tried to read pochi://skills/developer-updates/references/FORMAT.md but didn't have the resolved path context):

🤖 Generated with Pochi | Task

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 27.94%. Comparing base (056f686) to head (53a3b7d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1776   +/-   ##
=======================================
  Coverage   27.93%   27.94%           
=======================================
  Files         753      753           
  Lines       76588    76598   +10     
  Branches     3076     3080    +4     
=======================================
+ Hits        21393    21402    +9     
- Misses      54923    54924    +1     
  Partials      272      272           

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