support multiple branches during init

[skarim]

Revamp gh stack init onboarding experience

Redesign the init flow to be more intuitive for first-time users: implicit branch adoption, automatic prefix detection, a simplified interactive flow, and scenario-aware post-init output.

Changes

• Implicit adopt (cmd/init.go): Existing branches are now adopted automatically when passed as args — no flag needed. Missing branches are created. Two-pass validation ensures no branches are created if a later arg fails.
• Prefix detection: Drop the upfront "Set a branch prefix?" prompt. Prefix is now derived from branch names using last-slash split (e.g. feat/a feat/b → prefix feat). Interactive path confirms with Y/n; args path sets it silently.
• Interactive flow: Print a hint about multi-branch args. On a non-trunk branch, show a Select prompt ("Use current branch" / "Create new"). On trunk, prompt for branch name directly.
• "What's next" output: Replace the 2-line post-init message with a scenario-aware block — fresh stacks show add/view/submit tips; adopted stacks show view/up/down/submit tips with PR count.
• Deprecate --adopt: Flag is hidden but still accepted. Prints a warning: "The --adopt flag is deprecated."
• Docs: Remove all --adopt references from README, CLI reference, FAQ, SKILL.md. Add "Turning Existing Branches into a Stack" section to workflows guide.

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

[Copilot]

Pull request overview

Revamps gh stack init to better support first-time onboarding and multi-branch initialization by implicitly adopting existing branches, detecting prefixes from branch names, simplifying the interactive flow, and improving post-init guidance.

Changes:

• Makes branch adoption implicit when branches already exist; creates missing branches with two-pass validation for arg parsing.
• Adds automatic prefix detection (from branch names) and updates interactive prompts accordingly; deprecates and hides --adopt.
• Reworks post-init output into a scenario-aware “What’s next” block and updates docs/examples across guides and references.

Show a summary per file

File	Description
skills/gh-stack/SKILL.md	Updates init examples to remove --adopt and document multi-branch init/adoption.
README.md	Refreshes gh stack init docs to describe implicit adoption and prefix detection.
internal/git/mock_ops.go	Re-indents MockOps fields (formatting/consistency).
docs/src/content/docs/reference/cli.md	Updates CLI reference text/examples to remove --adopt and reflect new behavior.
docs/src/content/docs/guides/workflows.md	Adds a new section explaining how to turn existing branches into a stack via init.
docs/src/content/docs/faq.md	Updates FAQ examples to remove --adopt and reflect the new workflow.
cmd/init.go	Implements new init flow: implicit adopt/create, prefix detection, deprecated --adopt, PR detection, and new output block.
cmd/init_test.go	Updates/expands tests for new init behaviors (implicit adopt, prefix detection, “what’s next” output).

Copilot's findings

Tip

Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

Comments suppressed due to low confidence (1)

cmd/init.go:483

• printWhatsNext prints Adopted %d branches using len(branches) whenever hasAdopted is true, even for mixed cases where some branches were newly created. This can mislead users about what actually happened. Consider adjusting the message to distinguish adopted vs created counts (or use more neutral wording like “Initialized stack” with a breakdown).

	// Success line
	if hasAdopted {
		cfg.Successf("Adopted %d %s: %s",
			len(branches), plural(len(branches), "branch", "branches"), chain)
	} else {
		cfg.Successf("Created stack: %s", chain)
	}

• Files reviewed: 7/8 changed files
• Comments generated: 2

[ktravers]

This was a fairly big PR to review, but the thorough, well organized and well described test coverage makes me confident to approve 😄

These changes also seem incredibly user-friendly and helpful, excited for folks to try them out!