Add terminal repo content browsing

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Repository improvement issue

Problem

The terminal repository explorer could list, filter, open, and clone repositories, but visitors still had to leave the site to inspect source structure or README context. This limited progress toward the site behaving like a terminal emulator for Jakob's GitHub repositories, especially systems-programming projects.

Implemented solution

• Added repo files <name|number> [path] / repo tree <name|number> [path] to fetch and render GitHub repository contents in terminal output.
• Added repo readme <name|number> to fetch README content through GitHub's public contents API and show a bounded terminal preview.
• Added reusable terminal helper functions for GitHub contents/readme API URLs, repo/path parsing, file-list formatting, and README formatting.
• Updated repository detail output, command help, and README documentation to advertise the new browsing commands.
• Added focused unit coverage for content URL building, target parsing, file listings, README truncation, and helper edge cases.

Acceptance criteria

• repo files wordlehelper lists root repository entries without leaving the terminal UI.
• repo tree jakobs-ls-remake src can inspect a directory path inside a GitHub repo.
• repo readme Unix-Permissions-Game prints a README preview in the terminal.
• Existing repo list/filter/detail/open/clone flows continue to work.

Verification

• Pre-push npm run check passed: lint, format check, unit tests with coverage, and Astro build.
• Test result: 69 tests passed; branch coverage 84.34% (threshold 83%).