feat: enable MCP server by default

Enable the MCP (Model Context Protocol) server feature by default. This
allows AI agents to interact with ADR repositories out of the box without
requiring users to rebuild with --features mcp.

Binary size impact: +2.5MB (6.4MB -> 8.9MB)
Dependency count: +~200 dependencies

Users who want to disable MCP can still build with --no-default-features.

Closes #164

Author: joshrotenberg

crates/adrs/Cargo.toml

@@ -13,7 +13,7 @@ name = "adrs"
 path = "src/main.rs"
 
 [features]
-default = []
+default = ["mcp"]
 mcp = ["rmcp", "schemars", "tokio"]
 mcp-http = ["mcp", "tokio-util", "axum"]