Merge Stacked PRs with merge command

[skarim]

This adds a new gh stack merge command built on GitHub's new async merge API, which can land multiple stacked PRs atomically. It merges some or all of a stack in one atomic operation: every PR up to and including the one you choose is merged into the base branch, and if any PR can't be merged, none are.

What's new

• Three ways to target a merge. No argument merges the current active local stack; a stack number merges a stack you don't have checked out (a purely remote operation); a PR number merges directly up to that PR.
• Interactive wizard. In a terminal, a three-step wizard lets you choose how far up the stack to merge (a bottom-anchored checkbox list, so everything below your selection is always included), pick the merge method (only the methods the repo allows, defaulting to your last-used one), and confirm — then a live view polls the merge to completion.
• Non-interactive / agent-friendly. With --yes or no TTY, the whole stack (or everything up to the given PR) merges without prompting, using your last-used merge method unless one is given via --merge / --squash / --rebase / --merge-method.
• Light pre-checks only. PRs are checked for being open and non-draft; GitHub evaluates branch protection and repository rules at merge time and reports the failure reason back. Because the merge is atomic, a failure lands nothing.
• Merge queues are detected and rejected. If the base branch uses a merge queue, the command stops before doing anything and points you to gh pr merge — the stack merge lands directly, not through a queue. Admin bypass is likewise not applied by the stack-merge API.

Changes

The command

• cmd/merge.go — the new gh stack merge command: resolves the target stack (current branch / stack number / PR number), computes the mergeable candidates (open, non-draft, contiguous from the bottom), and runs either the interactive wizard or the headless submit-and-poll path. Registered in cmd/root.go.

GitHub client

• internal/github/merge_async.go — an async stack-merge client over the REST API: MergeStackAsync (submit) and GetAsyncMergeResult (poll) on go-gh's REST client, returning a status enum (pending / merged / failed); plus RepoMergeConfig (allowed methods and last-used default, via GraphQL), BaseBranchPolicy (merge-queue detection), and PRTitles (batch PR titles for the picker). Added to ClientOps and MockClient.

Wizard TUI

• internal/tui/mergeview/ (new package) — the Bubble Tea wizard: a stepper banner, the bottom-anchored PR picker with a scrolling viewport, the allowed-method picker, the confirm step, and the live progress view.

Docs

• README.md, docs/ CLI reference and workflows guide, skills/gh-stack/SKILL.md, and AGENTS.md — document the command, its targeting and merge-method behavior, and the merge-queue limitation.

Testing

• Unit tests for the command paths (cmd/merge_test.go), the async-merge client including status decoding and error mapping (internal/github/merge_async_test.go), and the wizard model (internal/tui/mergeview/model_test.go).
• Live-validated against a real repo: headless and interactive merges, partial (up-to-a-PR) merges, atomic rollback on a failing/conflicting PR, and merge-queue rejection.
• go test -race -count=1 ./... and go vet ./... are clean, and the changed files pass gofmt.

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