Reject --disable-promotion in dune fmt and dune promotion apply

[Copilot]

• fixes dune fmt ignores --disable-promotion flag #5946

dune fmt silently ignored --disable-promotion (overriding it internally with Automatically), and dune promotion apply accepted it without complaint — both are nonsensical combinations since these commands exist specifically to perform promotion.

Changes

• bin/common.ml / bin/common.mli: Expose a promote accessor on Common.Builder so callers can inspect the parsed promotion flag before it is overridden.
• bin/fmt.ml: Raise a User_error if --disable-promotion is passed, directing users to --preview instead.
• bin/promotion.ml: Raise a User_error if --disable-promotion is passed to dune promotion apply.
• test/…/promote/disable-promotion.t/run.t: Add cram test cases covering both rejection paths.

$ dune fmt --disable-promotion
Error: The --disable-promotion flag is not applicable to `dune fmt`. Use
--preview to display formatting changes without applying them.
[1]

$ dune promotion apply --disable-promotion
Error: The --disable-promotion flag is not applicable to `dune promotion
apply`.
[1]

Note: the check also fires when --disable-promotion's backing env var DUNE_DISABLE_PROMOTION is set, since both map to the same Promote.Never value.

[Alizter]

@copilot Which issue does this fix? Please update the PR description.

[Copilot]

@copilot Which issue does this fix? Please update the PR description.

This fixes #5946.

[Leonidas-from-XIV]

Seems like a sensible fix to me.