Diagnostic for unreachable targets

Detect targets that cannot be reached from the default goal or from any .PHONY target through the prerequisite graph. These are likely dead rules that could be cleaned up. This would need to account for pattern rules and targets that might be invoked directly from the command line.

No file, test, or entry point is named in the issue. Start by locating how target and prerequisite graphs are represented, then determine how default and .PHONY roots, pattern rules, and command-line targets should be handled. Done means unreachable targets are diagnosed without flagging targets that may be invoked directly.

Written by the indexing model from the issue text.