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/body-source.ts

@@ -0,0 +1,74 @@
+import { readFileSync } from "node:fs";
+import { isAbsolute, resolve } from "node:path";
+import type { CliDeps } from "./deps.js";
+import { axiError } from "./errors.js";
+import { flagValue } from "./flags.js";
+
+/**
+ * Resolve the body text a mutation should send from its `--body`/`--body-file`
+ * flags. Shared by every command that accepts a body (issue create, issue
+ * comment, and the edit/close commands that follow).
+ *
+ * The two flags are mutually exclusive: accepting both and picking a winner
+ * would silently discard text the caller meant to send.
+ */
+export function resolveBodySource(
+  deps: CliDeps,
+  flags: Record<string, string | true>,
+  command: string,
+): string | undefined {
+  const body = flagValue(flags, "--body");
+  const path = flagValue(flags, "--body-file");
+
+  if (body !== undefined && path !== undefined) {
+    throw axiError(
+      "Flags --body and --body-file are mutually exclusive",
+      "VALIDATION_ERROR",
+      bodyFlagSuggestion(command),
+    );
+  }
+  if (body !== undefined) {
+    return body;
+  }
+  if (path !== undefined) {
+    return readBodyFile(deps, path, command);
+  }
+  return undefined;
+}
+
+/** As {@link resolveBodySource}, for the commands where a body is mandatory. */
+export function requireBodySource(
+  deps: CliDeps,
+  flags: Record<string, string | true>,
+  command: string,
+): string {
+  const body = resolveBodySource(deps, flags, command);
+  if (body === undefined) {
+    throw axiError(
+      `${command} requires --body <text> or --body-file <path>`,
+      "VALIDATION_ERROR",
+      bodyFlagSuggestion(command),
+    );
+  }
+  return body;
+}
+
+function bodyFlagSuggestion(command: string): string[] {
+  return [
+    `Run \`gitea-axi ${command} --body <text>\` or \`gitea-axi ${command} --body-file <path>\``,
+  ];
+}
+
+function readBodyFile(deps: CliDeps, path: string, command: string): string {
+  const absolute = isAbsolute(path) ? path : resolve(deps.cwd, path);
+  try {
+    return readFileSync(absolute, "utf8");
+  } catch (error) {
+    const reason = error instanceof Error ? error.message : String(error);
+    throw axiError(
+      `Cannot read --body-file ${path}: ${reason}`,
+      "VALIDATION_ERROR",
+      bodyFlagSuggestion(command),
+    );
+  }
+}