feat: add issue create and comment (task 0004)

Introduce the first mutations, along with the shared machinery the later
issue and PR mutation slices reuse.

- `issue create` with --title/--body/--body-file/--assignee/--label/
  --milestone/--fields, emitting `issue: { number, title, state, url }`
- `issue comment <n>`, echoing the created comment from the POST response
  with the body cleaned and truncated at 800 chars
- body-source resolution (--body vs --body-file), label and milestone
  name->ID lookup, repeatable flags, and the `joined`/`selectExtraFields`
  field extractors

Label lookup pages until exhausted, since a repo with more labels than one
page would otherwise fail to resolve a valid name. The end-to-end tier seeds
a mixed-case label and milestone and passes both in a different case, so the
case-insensitive lookup is verified against live Gitea rather than only
against fixtures.

src/fields.ts

@@ -1,3 +1,4 @@
+import { axiError } from "./errors.js";
 import { relativeTime } from "./time.js";
 
 export interface ExtractContext {
@@ -31,6 +32,26 @@ export function lowercased<T>(name: string, path: string = name): FieldDef<T> {
   };
 }
 
+/**
+ * Join a field holding an array of objects (labels, assignees) into a single
+ * string of each element's `key` property.
+ */
+export function joined<T>(name: string, path: string, key: string): FieldDef<T> {
+  return {
+    name,
+    extract: (raw) => {
+      const value = pluckPath(raw, path);
+      if (!Array.isArray(value)) {
+        return "";
+      }
+      return value
+        .map((item) => pluckPath(item, key))
+        .filter((item): item is string => typeof item === "string" && item.length > 0)
+        .join(", ");
+    },
+  };
+}
+
 export function relativeTimeField<T>(name: string, path: string): FieldDef<T> {
   return {
     name,
@@ -41,6 +62,48 @@ export function relativeTimeField<T>(name: string, path: string): FieldDef<T> {
   };
 }
 
+/**
+ * Resolve a comma-separated `--fields` value against the extra fields a command
+ * offers on top of its defaults. Unknown names are a `VALIDATION_ERROR` naming
+ * what is available, since a silently ignored field would look like a command
+ * that returned nothing for it.
+ */
+export function selectExtraFields<T>(
+  value: string | undefined,
+  registry: Record<string, FieldDef<T>>,
+  command: string,
+): FieldDef<T>[] {
+  if (value === undefined) {
+    return [];
+  }
+  const available = Object.keys(registry);
+  const suggestion = [
+    `Run \`gitea-axi ${command} --fields <a,b,c>\` with any of: ${available.join(", ")}`,
+  ];
+  const selected: FieldDef<T>[] = [];
+  const seen = new Set<string>();
+  for (const raw of value.split(",")) {
+    const name = raw.trim();
+    if (!name) {
+      continue;
+    }
+    const field = registry[name];
+    if (!field) {
+      throw axiError(
+        `Unknown --fields name: ${name} (available: ${available.join(", ")})`,
+        "VALIDATION_ERROR",
+        suggestion,
+      );
+    }
+    if (seen.has(name)) {
+      continue;
+    }
+    seen.add(name);
+    selected.push(field);
+  }
+  return selected;
+}
+
 export function extractRow<T>(
   raw: T,
   fields: FieldDef<T>[],