feat: gate experimental commands behind build-time visibility flag

Introduce cmd/entire/cli/experimental: a build-time gate (Visible, stamped false by GoReleaser via ldflags) that shows a curated set of maturing commands in developer builds and hides them in shipped releases. When visible, they are filed under a single 'Experimental commands:' cobra group in help; when hidden they carry no GroupID so release help is unchanged and cobra never references an unregistered group.

Gated (visible in dev, hidden in release, always runnable): tokens, import, review, investigate, blame, why, search, experts, runner (root) and checkpoint policy. Infra/plumbing, deprecated shortcuts, and aliases are untouched and stay always-hidden.

• .goreleaser.yaml stamps experimental.Visible=false (nonprod builds only git-remote-entire, so no stamp needed there).
• Add 'mise run build' (plain go build; experimental visible).
• Update existing tests that asserted these commands were Hidden to assert the experimental grouping instead.