trunk command (#108)

* 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

* address review comments

* increment skill version

prune merged branches (#94)

* prune merged branches

* interactively prompt for prune

* delete remote tracking ref too

* disable selecting merged branches in TUIs

* include full list (including merged PRs) in PUT request to stacks API

* add prune to docs

* addressing review comments

* increment skill file version

improve agent friendliness of view --json (#80)

* return exit codes instead of interactive prompt for view json mode

* increment skill file version

switch command (#51)

* switch cmd to interactively switch to another branch in the stack

* add switch to docs

* addressing review comments

* bump skill file version

* default to current branch

Initial release