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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