refactor the way we load and manage skills:
1. Move skill discovery/caching into SkillsManager and reuse it across
sessions.
2. Add the skills/list API (Op::ListSkills/SkillsListResponse) to fetch
skills for one or more cwds. Also update app-server for VSCE/App;
3. Trigger skills/list during session startup so UIs preload skills and
handle errors immediately.
This PR changes the length validation for SKILL.md `name` and
`description` fields so they use character counts rather than byte
counts. Aligned character limits to other harnesses.
This addresses #7730.
1. Skills load once in core at session start; the cached outcome is
reused across core and surfaced to TUI via SessionConfigured.
2. TUI detects explicit skill selections, and core injects the matching
SKILL.md content into the turn when a selected skill is present.
This change prototypes support for Skills with the CLI. This is an
**experimental** feature for internal testing.
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Co-authored-by: Gav Verma <gverma@openai.com>