Add external database directory support for network file systems

[felipecaldas]

Fixes SQLite WAL mode failures on Samba/CIFS shares by allowing users to store the graph database in an external local directory while keeping source code on network shares.

Problem:

• SQLite's WAL mode requires filesystem features not fully supported by network file systems
• Building graph on Samba/NFS shares fails with 'database is locked' error at PRAGMA journal_mode=WAL
• Developers using network shares cannot use code-review-graph

Solution:

• Add --data-dir CLI flag to store database in external location
• Extend multi-repo registry to remember per-repo data_dir configuration
• Implement resolution priority: registry > CRG_DATA_DIR env var > default .code-review-graph/
• All CLI commands and MCP tools automatically use the external location

Changes:

• code_review_graph/registry.py: Add set_data_dir() and get_data_dir_for_repo() methods
• code_review_graph/incremental.py: Modify get_data_dir() to check registry first
• code_review_graph/cli.py: Add --data-dir argument to build, update, status, watch, visualize, wiki, postprocess
• code_review_graph/tools/registry_tools.py: Fix cross_repo_search to use get_db_path() for custom locations
• tests/test_registry.py: Add TestSetDataDir class with 5 tests
• tests/test_incremental.py: Add TestDataDirRegistry class with 5 tests

Benefits:

• Eliminates database lock errors on network file systems
• Better performance (local disk vs network I/O)
• Clean separation of source code and index data
• Persistent configuration via registry
• 100% backward compatible
• All 28 MCP tools work automatically with custom data_dir

Usage:
cd /mnt/repos/my-project code-review-graph build --data-dir ~/.crg-graphs/my-project

Testing:

• All 83 tests pass (registry + incremental)
• Manual testing successful on test project
• Cross-repo search verified with custom data_dir
• MCP server integration confirmed