hive-forge: add # Errors docs on every verb's pub fn run (closes #816)

systemic gap argus flagged on PR #798 (timeline verb). every `pub fn
run` in hive-forge/src/verbs/*.rs lacked a `# Errors` block —
violates Rust API guidelines + obscures the failure surface for
operators reading the source.

uniform doc per verb category:
- pure GET + print verbs: "transport error from the Forgejo REST call
  + I/O error from stdout"
- body-from-file verbs (comment/comment_edit/issue_create/issue_edit/
  pr_create): adds 'I/O error from --body-file/stdin input'
- file-upload verbs (attach-issue, attach-comment): adds 'file
  read/exist check'
- pr_create: also mentions the --push shellout

23 `pub fn run` signatures touched. no behaviour change; pure
documentation sweep. cargo test green (38 tests).

hive-forge/src/verbs/diff.rs

@@ -30,6 +30,11 @@ pub struct Args {
     full: bool,
 }
 
+/// # Errors
+///
+/// Propagates any transport error from the Forgejo REST call
+/// (network unreachable, 4xx/5xx response, token missing/invalid)
+/// and any I/O error from writing the response to stdout.
 pub fn run(client: &Client, args: Args) -> Result<()> {
     let repo = client.repo();
     let diff = client.get_text(&format!("/repos/{repo}/pulls/{}.diff", args.number), "text/plain")?;