trunk command

[skarim]

Add gh stack trunk navigation command

Add a new navigation command that checks out the trunk branch of the
current stack.

The command is stack-aware: it requires the user to be on a branch that
is part of a stack, loads the stack metadata, and checks out s.Trunk.Branch.
If the user is already on the trunk branch, it prints a message and exits
without calling git checkout.

New files:

• cmd/trunk.go: TrunkCmd (cobra command) + runTrunk implementation
• cmd/trunk_test.go: 7 test cases covering happy path, already on
  trunk, from top of stack, not in a stack, checkout failure, custom
  trunk branch name, and positional argument rejection

Modified files:

• cmd/root.go: register TrunkCmd in the "nav" command group
• README.md: add gh stack trunk to the Navigation section
• docs/src/content/docs/reference/cli.md: add gh stack trunk
  reference section

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[Copilot]

Pull request overview

Adds a new stack-aware navigation command, gh stack trunk, to quickly switch from any branch in a stack to that stack’s configured trunk branch.

Changes:

• Introduces gh stack trunk Cobra command implementation and stack-aware checkout logic.
• Adds test coverage for trunk navigation behavior and argument validation.
• Documents the new command in README navigation examples and the CLI reference docs, and registers it under the Navigation command group.

Show a summary per file

File	Description
README.md	Lists gh stack trunk in the Navigation command examples.
docs/src/content/docs/reference/cli.md	Adds a new reference section documenting gh stack trunk.
cmd/trunk.go	Implements the new trunk navigation command and its checkout behavior.
cmd/trunk_test.go	Adds command-level tests for trunk navigation scenarios.
cmd/root.go	Registers trunk under the existing nav command group.

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• Files reviewed: 5/5 changed files
• Comments generated: 3