mcp: support SEMCODE_GIT_REPO environment variable

[groeck]

Add SEMCODE_GIT_REPO environment variable support for specifying the git repository path in semcode-mcp and semcode. This allows the MCP host process to provide the repository location via the environment when command line arguments cannot be easily modified.

The priority order is:

1. --git-repo command line argument
2. SEMCODE_GIT_REPO environment variable
3. Current directory (default)

Enable the env feature for clap in Cargo.toml to support the env attribute in the Args struct.

Add test code to verify that the environment variable is correctly supported and respects the priority order.

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