feat: add family sharing integration

[CodeKeanu]

Summary

• Add get_family_group tool to get family group membership information (members, roles, cooldown status, available slots)
• Add get_shared_library_apps tool to get games available through family sharing (grouped by owner, with exclusion reasons)
• Graceful handling for users not in a family group or with private profiles
• 14 unit tests with mocked responses

Changes

• New endpoint module: src/steam_mcp/endpoints/family_groups.py
• New test file: tests/test_family_groups.py
• Updated README (30 -> 32 tools)
• Updated CHANGELOG

Testing

• All existing tests pass (201 tests)
• New functionality tested with 14 unit tests

Checklist from Issue

• Family group membership retrieval
• Shared library listing with availability
• Graceful handling for non-family members
• Unit tests with mocked responses
• README and CHANGELOG updated

Closes #10

🤖 Generated with Claude Code

[CodeKeanu]

Code Review - Family Sharing Integration

Verdict: ✅ Approve

Strengths:

• Comprehensive test coverage (314 lines)
• Clean grouping logic for shared apps
• Proper 401/403 error handling

Suggestions (non-blocking):

• _resolve_steam_id() duplicated across 5 endpoint files - recommend follow-up issue to extract to BaseEndpoint
• owner_apps[:10] hardcoded - consider making configurable

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Review by @agent-ml-python-mcp-expert