fix: auth, signing, and CLI improvements

[Huerte]

Type

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

Changes

• ssh_utils.py: gitgo push no longer tries to convert HTTPS to SSH for non-GitHub hosts (Gitea, GitLab, private servers). It checks the remote hostname first. Only github.com gets the SSH conversion. This was the main reason push failed silently on private servers with a different account.
• ssh_utils.py: added is_agent_loaded(key_path) to check if a key is in the SSH agent. Also added get_remote_host(url) to extract the hostname from any remote URL.
• ssh_utils.py: ensure_known_host now accepts any hostname, not just github.com. Old ensure_github_known_host kept as an alias so nothing breaks.
• git_core.py: _get_signing_flags now uses the SSH signing key file from disk directly, and no longer crashes or skips signing based on the SSH agent state.
• account.py: sanitize_signing_config now checks if the SSH signing key file exists on disk, disabling commit signing gracefully if it is missing to prevent failures.
• push.py: when gitgo push is run with no arguments, both the default branch and the default message are now shown. Before, only one of them was printed.
• main.py: removed the letter aliases (-l, -s, -o, -d) from gitgo state. They caused confusion for new users who ran -s expecting something other than save.
• main.py: rewrote help text for push, sync, link, jump, resolve, undo, and state in plain language.
• state.py: removed alias resolution logic from state_operation to match the parser change.
• README.md: updated gitgo push section to document the both-defaults behavior with an example output. Updated gitgo state, sync, link, jump, resolve, and undo sections to match new help text. Removed the aliases note from state docs.

Checklist

• Tested locally and changes work as expected
• CHANGELOG.md updated under [Unreleased]
• README.md updated (if a command was added or changed)
• Tests added or updated (if applicable)
• No existing commands are broken (if they are, describe the impact above)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 88.93443% with 27 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pygitgo/auth/ssh_utils.py	72.41%	16 Missing ⚠️
src/pygitgo/commands/state.py	93.93%	10 Missing ⚠️
src/pygitgo/commands/push.py	75.00%	1 Missing ⚠️

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