unstack command

[skarim]

New Command: unstack
Adds the gh stack unstack (alias delete) command, which removes a stack from local tracking and deletes it on GitHub, with a --local flag to only remove local tracking. Handles error cases such as missing stack IDs and remote API failures, ensuring local state is not modified if there is a remote failure.

Also includes improvements to the init --adopt logic to record only open/draft PRs for adopted branches, ignoring closed/merged PRs, and removed the strict error that previously blocked adoption if a branch already had a PR.

• Implements half of https://github.com/github/pull-requests/issues/23868
• Implements https://github.com/github/pull-requests/issues/24004

[Copilot]

Pull request overview

Adds a new gh stack unstack command for tearing down stacks (with a delete alias), and adjusts adoption behavior to better handle existing PRs while expanding documentation and test coverage.

Changes:

• Introduces gh stack unstack / gh stack delete to delete a stack on GitHub and remove local tracking (or --local for local-only).
• Adds GitHub client support for deleting stacks (DeleteStack) plus command-level tests for success and failure cases.
• Updates init --adopt to associate only open/draft PRs and to no longer hard-fail when an adopted branch already has a PR.

Show a summary per file

File	Description
skills/gh-stack/SKILL.md	Documents unstack for restructuring stacks.
README.md	Adds user-facing docs for gh stack unstack / delete.
internal/stack/stack_test.go	Updates test section header comment for RemoveStackForBranch.
internal/github/client_interface.go	Adds DeleteStack to ClientOps.
internal/github/mock_client.go	Implements DeleteStack on the mock client.
internal/github/github.go	Implements REST DeleteStack call.
docs/src/content/docs/reference/cli.md	Adds unstack reference documentation.
docs/src/content/docs/introduction/overview.md	Mentions stack teardown via unstack.
docs/src/content/docs/guides/workflows.md	Adds a workflow section for restructuring stacks using unstack.
docs/src/content/docs/faq.md	Updates FAQs to recommend unstack/init --adopt for restructuring/deleting.
cmd/unstack.go	New command implementation for remote+local (or local-only) stack deletion.
cmd/unstack_test.go	New tests covering unstack success paths and API error handling.
cmd/root.go	Registers the new unstack subcommand.
cmd/root_test.go	Extends subcommand registration expectations to include unstack.
cmd/init.go	Adjusts --adopt PR association logic (open/draft only) and removes prior hard failure.
cmd/init_test.go	Adds tests for the updated --adopt behavior with existing PRs.

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• Files reviewed: 16/16 changed files
• Comments generated: 4

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