feat: support pull requests

Support for pull requests, it's not standard in NIPs yet[1] but there must be
two clients that support it to be included in NIPs, n34 is one of them.

This support:
- Create a new PR `n34 pr new`
- Update PR tip `n34 pr update`
- View a PR `n34 pr view`
- List PRs `n34 pr list`
- Add `PR` and `PR-update` kinds as roots in `NostrClient::find_root`
- Support issuing statuses for PRs
- Support GRASP PR and PR-update
- Update the docs

[1] https://github.com/nostr-protocol/nips/pull/1966

docs/commands.md

@@ -1,5 +1,17 @@
 # Command-Line Usage
 
+## Philosophy
+
+`n34` emphasizes simplicity by exclusively handling Nostr tasks, requiring
+users to manage Git operations themselves. For instance, it won't merge or
+rebase patches into a branch but will prompt you to specify where to write them,
+allowing you to merge them using `git-am`. Similarly, when sending patches, it
+won't ask for the commits but will request the patch files instead.
+
+This approach enhances simplicity and freedom, giving you full control over
+your workflow. Additionally, it simplifies software maintenance by avoiding
+unnecessary complexity.
+
 ## Options
 
 The `n34` command-line tool accepts the following options: