fix: MCP server bd executable path resolution

Fixes issue where MCP tools failed with "bd executable not found"
when BEADS_PATH was set to command name instead of absolute path.

Changes:
- Remove BEADS_PATH=bd from plugin.json (use auto-detection)
- Enhance config validator to resolve command names via PATH
- Add comprehensive config validation tests (11 new tests)

The validator now accepts both:
  - Absolute paths: /usr/local/bin/bd
  - Command names: bd (resolved via shutil.which)

This makes the MCP server more robust and user-friendly while
maintaining backward compatibility.

All 90 tests pass.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

integrations/beads-mcp/src/beads_mcp/config.py

@@ -43,21 +43,27 @@ class Config(BaseSettings):
         """Validate BEADS_PATH points to an executable bd binary.
 
         Args:
-            v: Path to bd executable
+            v: Path to bd executable (can be command name or absolute path)
 
         Returns:
-            Validated path
+            Validated absolute path
 
         Raises:
             ValueError: If path is invalid or not executable
         """
         path = Path(v)
 
+        # If not an absolute/existing path, try to find it in PATH
         if not path.exists():
-            raise ValueError(
-                f"bd executable not found at: {v}\n"
-                + "Please verify BEADS_PATH points to a valid bd executable."
-            )
+            found = shutil.which(v)
+            if found:
+                v = found
+                path = Path(v)
+            else:
+                raise ValueError(
+                    f"bd executable not found at: {v}\n"
+                    + "Please verify BEADS_PATH points to a valid bd executable."
+                )
 
         if not os.access(v, os.X_OK):
             raise ValueError(