feat(skills): cap skill and spawn descriptions to one line

[TheGreatAxios]

Summary

• Cap every skill_search, use_skill, and spawn_agent registration description to one line of at most 160 characters (longest is 154), ending in a Guidelines pointer (Tool choice for skills, Orchestration for spawn)
• Truncate skill catalog descriptions to their first line at discovery and in skill_search result rows, so multi-line text renders as a single - name: desc line
• Cover the caps with new tests in tests/unit/skills.test.ts; prompt snapshots stay names-only and unchanged

Critic follow-ups

• BLOCKING fixed: slash-picker bypass in src/plugins/skill-commands.ts applied the shared firstLine export, so a multi-line SKILL.md frontmatter description renders as a single slash-picker line; keeper test added alongside the skill_search mapper test in tests/unit/skills.test.ts
• SHOULD-FIX fixed: dropped the dangling 'See Guidelines: Tool choice.' pointer from the two worker copies only (workerSkillSearchDefinition, workerUseSkillDefinition); primary copies keep it; all four stay one line within 160 (worker-search 125, worker-use 81)

Verification

• bun run typecheck passes (exit 0; uses a worktree-local install since the symlinked install misses a vendor dep, pre-existing on clean origin/main)
• bun run build passes (exit 0, via pre-commit hook)
• bun test tests/unit/skills.test.ts tests/unit/skill-commands.test.ts tests/unit/lexicon-skill.test.ts tests/unit/corbits-skills-catalog.test.ts tests/unit/telemetry-product-events.test.ts passes: 74 pass, 0 fail
• bun run lint passes (oxfmt and oxlint clean)

Fixes CL-8854

[linear-code]

CL-8854