fix(skills): expose shared expert skills in chat

[Legimity]

Summary

• Allow shared-expert viewers to read the expert's skill catalog and skill details while keeping mutation endpoints owner-only.
• Load shared experts' configured skills in the chat composer.
• Add backend and frontend regression coverage for shared skill access.

Fixes #495

Target branch

• Base is develop (feature / fix — default)
• Base is main (release/* or hotfix/* only)

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Refactor / chore
• Release / hotfix

Test plan

• uv run pytest tests/integration/test_agents_shared.py::test_peer_cannot_mutate_shared_agent_workspace_or_skills -q

• uv run ruff check src/octop/api/routers/skills.py tests/integration/test_agents_shared.py

• uv run ruff format --check src/octop/api/routers/skills.py tests/integration/test_agents_shared.py

• npx vitest run src/pages/Experts/components/sharedExpert.test.ts

• npx prettier --check src/pages/Chat/index.tsx src/utils/sharedExpert.ts src/pages/Experts/components/sharedExpert.test.ts

• npx eslint src/pages/Chat/index.tsx src/utils/sharedExpert.ts src/pages/Experts/components/sharedExpert.test.ts

• npm run build

• make all passes locally

• Added/updated tests

Checklist

• Updated CHANGELOG.md (if user-facing)
• README / docs updated (not needed)