support skill-scoped hooks

[abhinav-oai]

Why

Skills can already be distributed inside plugins, but today hooks only come from settings files and plugins. This adds the same frontmatter-based hook model to skills so a skill can carry behavior that only applies while that skill is active.

What

• Parse hooks from skill frontmatter alongside existing skill metadata.
• Build a turn-local hook overlay from the explicitly active skills and feed it through the existing hook runtime.
• Add HookSource::Skill across protocol, analytics, and hook UI surfaces.
• Skip SessionStart hooks declared by skills with a warning because skills activate after thread start.
• Add coverage for parsing frontmatter hooks and for PreToolUse hooks applying only on the turn where the skill is active.

Verification

• cargo test -p codex-hooks
• cargo test -p codex-core-skills parses_hooks_from_skill_frontmatter
• cargo test -p codex-core --test all suite::hooks::skill_pre_tool_use_blocks_only_while_skill_is_active
• cargo test -p codex-app-server-protocol

Documentation

The public skills documentation on developers.openai.com/codex should be updated to describe skill-frontmatter hooks and note that SessionStart is not meaningful for skills.