Respect repository default branch in gh stack link

[Copilot]

gh stack link hardcoded --base to main, so omitting the flag retargeted the bottom PR of the stack to main regardless of the repository's actual default branch. It now resolves the default branch dynamically, matching gh stack init.

Changes

• cmd/link.go: --base now defaults to "". When unset, runLinkCreateOrUpdate resolves the bottom base via git.DefaultBranch() (the same path as init), erroring with Use --base to specify the base branch if it can't be determined. Add-mode is unaffected since it ignores --base by design.
• cmd/link_test.go: Added git.SetOps mocks to existing PR-number/URL create-path tests (previously leaning on real git ops), plus focused tests for retargeting an existing bottom PR to the default branch, creating a bottom PR on it, the unresolvable-default error, and explicit --base bypassing resolution.
• Docs (README.md, docs/reference/cli.md, skills/gh-stack/SKILL.md): updated the --base default description.

Behavior

# repo default branch is "develop"
gh stack link 10 20 30      # bottom PR now targets develop (was: main)
gh stack link --base main 10 20 30   # explicit base still honored

• Fixes gh stack link does not respect the default branch of a repository #260