add: offer to initialize a stack automatically

[skarim]

• Closes Allow gh stack add to initialize a stack when current branch is not part of a stack #275

When gh stack add runs from a branch that is not part of a stack, it now offers to initialize a new stack instead of failing. Explicit and auto-generated branch names become the first layer; without a name, the existing init flow handles branch selection and prompting.

The shortcut reuses runInit, preserves commit flags, and leaves declined confirmations and non-interactive behavior unchanged. init now also restores a missing local trunk from the selected remote before branch creation, covering repositories where the default branch was renamed locally.

README and CLI reference documentation are updated along with regression coverage for both flows.

[Copilot]

Pull request overview

Adds an interactive fallback that initializes a stack when gh stack add runs outside one.

Changes:

• Reuses runInit from add, preserving branch and commit options.
• Restores missing local trunk branches from remotes.
• Updates documentation and regression tests.

Show a summary per file

File	Description
README.md	Documents automatic initialization flow.
docs/src/content/docs/reference/cli.md	Updates CLI reference.
cmd/utils.go	Adds optional stack lookup.
cmd/init.go	Restores missing local trunks.
cmd/init_test.go	Tests trunk restoration.
cmd/add.go	Adds initialization prompt and flow.
cmd/add_test.go	Tests initialization scenarios.

Review details

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• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Balanced