docs: add comprehensive help output

[dwilding]

This PR expands the output of gimmegit -h with an essentially complete summary of how the gimmegit command works. I've duplicated the help output in the README and added an automatic check that the README is up-to-date.

[Copilot]

Pull request overview

This PR adds comprehensive help documentation to gimmegit, expanding the help output from a basic usage summary to a detailed guide covering all features and options.

Key changes:

• Expanded help text from ~20 lines to ~92 lines with detailed sections for usage, arguments, options, and examples
• Updated error message wording for clarity ("contains" instead of "has")
• Corrected capitalization of "Git" in documentation

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/gimmegit/_help.py	Complete rewrite of help text with comprehensive documentation including usage patterns, directory structure, branch mapping, options, and examples
src/gimmegit/_cli.py	Updated error message to use "contains" instead of "has" for clarity
tests/functional/test_clone.py	Updated test expectation to match the new error message wording
README.md	Standardized capitalization of "Git" and improved formatting of multi-line command examples

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[dwilding]

@copilot let's add a step to the "checks" CI that checks that the help output in README.md matches the output of uv run gimmegit -h. Here's the high-level approach I'd like you to take:

1. Look for the following marker in README.md:

   # Command reference
   
   ```text
   

2. Extract the subsequent text, until the closing triple backtick.

3. Diff the extracted text against the output of uv run gimmegit -h, failing the workflow step if there's a difference.

[Copilot]

@dwilding I've opened a new pull request, #72, to work on those changes. Once the pull request is ready, I'll request review from you.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.