Add machine-readable `--json` output to reporting commands

[AshwinUgale]

Context

coverage, drift, mine, and select print human-formatted text only. The CLI already advertises a stable exit-code contract for automation, and most of these commands already build a to_dict() report internally — so the data is right there, just not emitted in a machine-readable form.

Task

Add a --json flag to the reporting commands (coverage, drift, mine, select) that prints the underlying report dict as JSON to stdout, suppressing the human-formatted text under the flag.

Acceptance criteria

• JSON goes to stdout; diagnostics/warnings stay on stderr.
• Exit codes unchanged (the existing contract still holds).
• One test per command asserting the output parses as JSON with the expected keys.

Pointers

• _cmd_coverage, _cmd_drift, _cmd_mine, _cmd_select in src/goldset/cli.py

Help wanted

Scoped and mechanical, but touches four commands — a good second contribution. Can be split into one command per PR if a newcomer prefers a smaller first bite.