fix: resolve unconditional fall-through bugs across core commands

[Huerte]

Type

• Bug fix
• New feature
• Documentation
• Refactor / internal

Overview

Fixed a bunch of control-flow bugs found during the audit. The main issue was commands running downstream steps (like git add .) unconditionally even when the parent step was skipped or failed.

Changes

• gitgo link: split into two explicit paths so we don't accidentally run git add . on existing repos with unwanted files.
• gitgo push --select: added a skip_staging flag so it actually respects your interactive selection instead of committing everything anyway.
• gitgo user login: added proper error raising for bad emails and missing ssh-keygen instead of just crashing.
• gitgo user logout: inverted the order so we try to delete files first. this stops your git config from being wiped if the file deletion fails.
• gitgo jump: wrapped the pull command in an else block so it stops trying to pull from main when you just created a brand new local branch.

How to Test

1. pip install -e ".[dev]"
2. Run gitgo push --select and pick a few files. Verify that only those files got committed.
3. Run gitgo jump new-test-branch. Verify it checks out the branch without hanging to pull updates.
4. Expected result: Everything works cleanly without staging files you didn't ask for.

Checklist

• I tested my changes locally and they work
• I updated CHANGELOG.md under the [Unreleased] section
• I updated README.md (if I added or changed a command)
• I added or updated tests for my change (if applicable)
• My change does not break any existing commands