Add MultiRepo for multi-repository analysis

[tnm]

Summary

Add MultiRepo class for unified analysis across multiple repositories - ideal for microservices, frontend/backend splits, and team projects.

Proposed API

from kit import MultiRepo

# Initialize with multiple repo paths
repos = MultiRepo([
    "~/code/frontend",
    "~/code/backend",
    "~/code/shared-lib",
])

# Access
repos.names                              # list of repo names
repos.repos                              # Dict[str, Repository]
repos["frontend"]                        # Get single Repository
for name, repo in repos: ...             # Iterate

# Text search
repos.search("handleAuth")
repos.search("TODO", file_pattern="*.py", max_results_per_repo=10)

# Semantic search (most valuable for multi-repo!)
repos.index_all(embed_fn=model.encode)   # Build indexes once
repos.search_semantic("authentication and session management", top_k=10)

# Symbol operations  
repos.find_symbol("UserModel", symbol_type="class")
repos.extract_all_symbols(symbol_type="function")

# Auditing & summaries
repos.audit_dependencies()               # Python, JS, Rust, Go
repos.summarize()                        # File counts, languages

# File access
repos.get_file_content("frontend", "src/App.tsx")

CLI Commands

kit multi search ~/frontend ~/backend -q "handleAuth"
kit multi symbols ~/frontend ~/backend -n "UserModel" -t class
kit multi deps ~/frontend ~/backend ~/shared-lib
kit multi summary ~/project-*

Changes

• src/kit/multi_repo.py - Core MultiRepo class (~400 lines)
• src/kit/cli.py - CLI commands
• tests/test_multi_repo.py - 26 tests
• docs/src/content/docs/api/multi-repo.mdx - Full API docs
• Brief references in README and usage guide

Test plan

• 26 unit tests passing
• ruff check passing
• mypy passing
• Manual testing with real repos