feat(cli): automatically enter workspace from but branch new

[davidpdrsn]

Just but branch new. Still missing the other commands that create new branches like commit and move.

[Copilot]

Pull request overview

This PR updates the but CLI and supporting workspace/TUI logic so that, in single-branch mode, creating additional (unstacked) branches transitions you into workspace mode automatically, while also improving correctness around applying already-applied branches outside a managed workspace.

Changes:

• Refactors legacy branch new operations and adjusts single-branch-mode behavior to enter workspace mode when creating additional independent branches.
• Extends workspace branch-apply and reference-creation logic to support single-branch-mode/workspace transitions (including explicit ordering metadata when needed).
• Updates/expands CLI and legacy TUI tests/snapshots, including an SBM label in the status TUI hot bar.

Reviewed changes

Copilot reviewed 18 out of 21 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
crates/but/tests/fixtures/scenario/single-branch-mode.sh	Adds a dedicated fixture for single-branch-mode scenarios where main and origin/main align.
crates/but/tests/but/command/push.rs	Updates expected output snapshot formatting.
crates/but/tests/but/command/pull.rs	Adjusts branch creation tests to specify explicit stack relationships (--above).
crates/but/tests/but/command/commit.rs	Adjusts branch creation in tests to specify --above for stacking.
crates/but/tests/but/command/branch/new.rs	Reworks single-branch-mode branch-new tests and adds coverage for SBM→workspace transitions.
crates/but/src/error.rs	Tweaks CliError::into_internal to drop BadInput hints for internal-error conversion.
crates/but/src/command/legacy/status/tui/tests/utils.rs	Drops an exclusive guard before App::new to avoid conflicting guard acquisition in tests.
crates/but/src/command/legacy/status/tui/tests/snapshots/shows_single_branch_mode_label_in_hot_bar_001.svg	Adds snapshot for SBM label display in the hot bar.
crates/but/src/command/legacy/status/tui/tests/snapshots/marks_spanning_checkouts_are_refused.svg	Updates snapshot rendering for revised error box/layout.
crates/but/src/command/legacy/status/tui/tests/snapshots/commit_from_unstaged_changes_to_new_branch_checks_out_branch_in_single_branch_mode_final.svg	Updates snapshot to reflect hot bar layout changes and SBM label.
crates/but/src/command/legacy/status/tui/tests/single_branch_mode.rs	Adds a TUI test asserting SBM label rendering.
crates/but/src/command/legacy/status/tui/tests/mod.rs	Registers the new SBM TUI test module.
crates/but/src/command/legacy/status/tui/render.rs	Renders an “SBM” label in the hot bar when in single-branch mode.
crates/but/src/command/legacy/status/tui/app/mod.rs	Tracks is_in_single_branch_mode in the TUI App and refreshes it on reload.
crates/but/src/command/legacy/push.rs	Switches legacy push handler to CliResult and propagates merged-upstream checks via ?.
crates/but/src/command/legacy/branch/new.rs	Refactors new-branch operations (stacked vs unstacked) and implements SBM→workspace entry behavior.
crates/but-workspace/tests/workspace/branch/apply_unapply.rs	Updates test helper options to include the new apply option field.
crates/but-workspace/src/branch/create_reference.rs	Enhances ad-hoc positioning at workspace base by requiring/persisting branch ordering metadata when necessary.
crates/but-workspace/src/branch/apply.rs	Adds allow_applying_already_applied_branch_when_outside_workspace option and updates apply behavior accordingly.
crates/but-debug/src/command/workspace.rs	Uses ..Default::default() to pick up new apply options safely.
crates/but-api/src/branch.rs	Wires the new apply option into the API call site (defaulting to false).

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