feat(cli): add et git create-branch command

[seb4stien]

Summary

Adds et git create-branch (aliased et git cb), which creates and
switches to a git branch named after the current task's Jira issue,
following Canonical's PR branch naming convention:
type/scope-short-description-jirakey (e.g. feat/tcp-wildcard-sni-support-isd-1234).

Details

• Resolves the ticket via the active workspace's linked issue, or a
  -j/--jira KEY override (matching the existing et jira convention).
• Prints the issue's clickable link and summary before any prompting.
• Proposes a branch type from the issue's type/labels (Bug→fix,
  Story→feat, Task→chore; a documentation label always wins and
  defaults to docs), letting the user accept it or pick a different one
  from the fixed feat/fix/docs/chore/test/ci list — this becomes the
  branch's type/ prefix, which isn't otherwise freely editable.
• Proposes the scope-short-description segment as a slug of the issue
  summary, freely editable at the prompt.
• Appends the resolved Jira key (lowercased) as the trailing identifier,
  which is never editable.
• Errors clearly if not run inside a git repo, or if a local branch with
  the computed name already exists.
• Creates the branch from the current HEAD and switches to it
  (git checkout -b).

New/changed files

• src/et/jira.py: new fetch_issue_basis (summary/type/labels).
• src/et/git_branch.py (new): pure branch-naming logic + git subprocess
  helpers + create_branch_interactive orchestrator.
• src/et/cli.py: new git Typer group with create-branch/cb.
• README.md / CHANGELOG.md: documented the new command.
• Tests: tests/test_git_branch.py (new), plus additions to
  tests/test_jira.py and tests/test_cli.py.

Testing

• just lint / just static / just test all pass (381 tests, 90.5%
  coverage).