Ensures that tags within each repository are stored in alphabetical order (a-z) in the .gitopolis.toml configuration file. Tags are now sorted at the point of mutation when they are added to a repository.

This complements the repository sorting implemented previously and provides a consistent alphabetical ordering throughout the configuration.

Implementation:
- Sort tags immediately after adding them by calling `repo.tags.sort()` in the `tag()` method
- Follows same mutation-point sorting pattern as repository sorting
- Existing test `list_long` updated to expect sorted tag order

Test coverage:
- Added end-to-end test `tags_stored_alphabetically()` that adds tags in reverse order (zulu, alpha) and verifies they are stored sorted (alpha, zulu)
- Updated `list_long` test expectation from "some_tag,another_tag" to "another_tag,some_tag"

Prompts:
- lovely jubbly. now make tag sorting stable

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Co-Authored-By: Claude <noreply@anthropic.com>
Fixes #225

Changed repository and tag sorting to be case-insensitive by converting
strings to lowercase before comparison. This provides more intuitive
alphabetical ordering where "alpha", "Beta", "zulu" sorts correctly
regardless of capitalization.

Implementation:
- Updated `Repos::add_repo()` to use `sort_by()` with `to_lowercase().cmp()`
- Updated `Repos::tag()` to use `sort_by()` with `to_lowercase().cmp()`

Test changes (TDD):
- Extended `repos_stored_alphabetically` test to include mixed-case "Beta_repo"
  - Expected order: alpha_repo < Beta_repo < zulu_repo (case-insensitive)
- Extended `tags_stored_alphabetically` test to include mixed-case "Beta" tag
  - Expected order: alpha < Beta < zulu (case-insensitive)

All 76 tests pass.

Prompts:
- expand the repo/tag sort tests to show they are case-insensitive
- [Request interrupted by user for tool use]um, that's not what I said, I said test for *insensitive*. I know it'll fail. TDD. duh.
- [Request interrupted by user for tool use]ALL the prompts, as per instructions. goldfish

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

#225

Updated three unit tests in gitopolis_tests.rs to include mixed-case
examples, verifying that repository sorting is case-insensitive throughout
the system.

Also fixed the `list()` method in gitopolis.rs to use case-insensitive
sorting, which was previously using case-sensitive sorting and causing
the updated tests to fail.

Changes:
- list_returns_repos_sorted_alphabetically: Added Beta_repo to verify alpha < Beta < zebra
- list_with_tag_filter_returns_sorted_repos: Added Beta_repo with backend tag
- tags_long_repos_sorted_within_tag: Added Beta_repo with backend tag
- Fixed Gitopolis::list() to use .to_lowercase().cmp() for case-insensitive sorting

All 76 tests pass.

Prompts:
- list_returns_repos_sorted_alphabetically list_with_tag_filter_returns_sorted_repos tags_long_repos_sorted_within_tag - all need prove case-insensitive sorting

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>