Remote unstack by stack number

[skarim]

gh stack unstack previously required the target stack to be tracked locally — running it on a stack number that wasn't checked out locally failed with stack #N is not tracked locally and never reached the API. This PR makes unstack <number> a remote-first operation (like gh stack link): the stack is unstacked directly through the GitHub API by its number, from anywhere in the repository, whether or not it's tracked locally. When the stack is also tracked locally, its local tracking is removed as well.

Behavior

• No argument (unchanged) — targets the active stack (the one containing the current branch), unstacking it on GitHub and removing local tracking; fails if you're not on a stack.
• <stack-number> tracked locally (unchanged) — unstacks on GitHub and removes local tracking.
• <stack-number> not tracked locally (new) — unstacks on GitHub by number, no local checkout required. A number that doesn't exist on GitHub reports stack #N not found on GitHub (exit 2).
• --local (unchanged) — only removes local tracking and never contacts GitHub; combining it with a number that isn't tracked locally is an error.

Changes

• cmd/utils.go — replace loadStackByNumber with a quiet lookupStackByNumber that reports "not tracked locally" without erroring, so the caller can fall back to the API (legacy-id backfill preserved).
• cmd/unstack.go — split runUnstack into a locally-tracked path and a remote-only path (runRemoteUnstack), sharing one Unstack API call and error mapping. A 404 is an idempotent success when there's local tracking to clean up, but a hard error in remote-only mode.
• Docs — README.md (which never documented the <stack-number> argument), docs/src/content/docs/reference/cli.md, faq.md, and skills/gh-stack/SKILL.md.

Testing

• Reworked cmd/unstack_test.go: replaced the old "not tracked → fail" test with remote-only cases for dissolved, 404, partial (200), 422, and --local on an untracked number; the local-path tests are unchanged.
• go test -race -count=1 ./... and go vet ./... are clean, and the changed files pass gofmt.

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[Copilot]

Pull request overview

Adds remote-first unstacking by stack number while reconciling local tracking when available.

Changes:

• Adds remote-only unstack handling and API error mapping.
• Refactors local stack-number lookup and legacy backfilling.
• Updates tests and user/agent documentation.

Show a summary per file

File	Description
cmd/unstack.go	Implements tracked and remote-only paths.
cmd/utils.go	Adds quiet local stack lookup.
cmd/unstack_test.go	Covers remote-only outcomes.
README.md	Documents numbered unstacking.
docs/src/content/docs/reference/cli.md	Updates CLI reference.
docs/src/content/docs/faq.md	Updates unstack guidance.
skills/gh-stack/SKILL.md	Updates agent instructions.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 5
• Review effort level: Medium

[talum]

👍🏼 LGTM