#8873: fix: document skill loading locations in system prompt

to avoid user-defined-skills not loading <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the Skills section of the agent system prompt (`src/agents/system-prompt.ts`) to explicitly document where OpenClaw loads skills from (bundled installation skills, managed `~/.openclaw/skills/`, workspace `{workspace}/skills/`, plugin-provided skills, and `config.skills.load.extraDirs`). This fits the codebase by aligning the system prompt guidance with the actual skill discovery paths implemented in the skills loader, which should reduce confusion when user-defined skills are not being picked up. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is additive, limited to prompt text, and matches the actual skill-loading sources used by the implementation (bundled/managed/workspace/plugin/extraDirs). No runtime logic is modified. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->