simplify unstack to only target the active stack

[skarim]

The unstack command previously accepted an optional [branch] positional argument to identify which stack to target. This was confusing because it wasn't clear that a branch name identifies a stack. This PR removes that argument so the command always targets the active stack — the one containing the currently checked-out branch.

Motivation

• Using a branch name to specify a stack is unintuitive — users don't think of stacks by branch name
• The default behavior (unstack current branch) was already the most common usage pattern
• A future change will add the ability to specify a stack by number, which is a clearer mental model

[Copilot]

Pull request overview

This PR simplifies gh stack unstack by removing the optional [branch] positional argument so the command always targets the active stack (the stack containing the currently checked-out branch), and updates user-facing documentation accordingly.

Changes:

• Removed the [branch] positional argument from gh stack unstack and made the command always resolve via the current branch.
• Updated CLI documentation (README, docs site reference, skill docs) to reflect the new invocation and behavior.
• Updated tests by removing branch-targeting coverage and adjusting the “correct stack removed” test to rely on current-branch resolution.

Show a summary per file

File	Description
skills/gh-stack/SKILL.md	Updates unstack docs to remove [branch] and clarify it targets the active stack.
README.md	Updates unstack docs/examples to remove [branch] and describe active-stack targeting.
docs/src/content/docs/reference/cli.md	Updates reference docs for unstack usage and behavior without [branch].
cmd/unstack.go	Removes positional arg handling and enforces no-args invocation.
cmd/unstack_test.go	Removes tests for branch-targeting and updates remaining tests for current-branch behavior.

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• Files reviewed: 5/5 changed files
• Comments generated: 2