#1551 git: add pull, fetch --tags, and cherry-pick methods

Building a release-preparation workflow against @swamp/git, three operations had no method and had to be reimplemented in a local extension model:

1. pull — no way to sync a branch. clone covers first acquisition only, so refreshing an existing checkout falls back to shelling out.
2. fetch --tags — no way to ensure tags are present locally. This matters when a branch must start at a tag rather than at a branch head.
3. cherry-pick — no method, including the conflict-abort path (cherry-pick --abort) needed to leave the working tree clean after a failed pick.

A startPoint already exists on branch, which pairs naturally with tag-based branching — the missing piece is just getting the tag locally and picking commits across branches.

Everything else (branch, commit, push, status) covered the use case well; the structured commitResult.sha in particular made it easy to chain a commit into a later step.

Upstream repository: https://github.com/swamp-club/swamp-extensions

Environment

• Extension: @swamp/git@2026.08.05.1
• swamp: 20260803.000157.0-sha.93a34c46
• OS: darwin (aarch64)
• Deno: 2.8.3
• Shell: /bin/zsh