breaking(but): remove but branch show in favor of but show

[slarse]

They are almost identical in functionality, but but branch show has three features that but show does not have:

1. The ability to fetch review information
2. The ability to check integration status
3. The ability to show branches not in the workspace

Point 3 is more of a bug in but show than a missing feature, it just shows an empty branch for any branch that is not applied. That will be fixed, and if there is any kind of demand we can reimplement review and merge checking.

This PR leaves a skeleton implementation of but branch show that simply tells the user to use but show instead.

[Copilot]

Pull request overview

This PR removes the legacy but branch show implementation and routes users to the newer but show command, keeping a hidden deprecated branch show subcommand that errors with guidance to preserve CLI compatibility.

Changes:

• Replace but branch show execution with an explicit error directing users to but show.
• Remove the legacy branch show implementation code and associated tests.
• Update CLI metrics naming to stop emitting a distinct BranchShow command metric.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
crates/but/src/lib.rs	Makes but branch show fail with a deprecation/removal message pointing to but show.
crates/but/src/args/branch.rs	Keeps a hidden branch show subcommand that accepts any trailing args for compatibility.
crates/but/src/args/metrics.rs	Removes the BranchShow metric command name variant.
crates/but/src/utils/metrics.rs	Maps branch show invocations to BranchList for metrics emission.
crates/but/src/command/legacy/branch/mod.rs	Removes the legacy show_branches entrypoint and module wiring.
crates/but/src/command/legacy/branch/show.rs	Deletes the legacy implementation of but branch show.
crates/but/tests/but/command/branch/mod.rs	Stops compiling the removed branch show test module.
crates/but/tests/but/command/branch/show.rs	Deletes tests that validated the old but branch show behavior.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

[Copilot]

Removing the branch/show.rs test module drops the only CLI coverage for the “show commits ahead for a branch” JSON behavior. Since but branch show is being replaced by but show, consider porting these tests to exercise but show <branch> (and --verbose if that’s the equivalent of the old --files) so the user-visible behavior remains covered.

Suggested change

	mod new;
	mod new;
	mod show;

[slarse]

This is actually a good find that might be relevant. The JSON output from but show does not contain commits_ahead. It's trivial to compute with e.g. but --json show <id> | jq '.commits | length', but still it's probably convenient to have there.

The JSON output of but show is also more verbose than but branch show in that it contains commit details (e.g. files) by default. Perhaps that will have a negative impact on agents. Some more consideration is necessary before making this switch.