fix: fall back to repo default_branch instead of hardcoded "main" (#1143)

* fix: fall back to repo default_branch instead of hardcoded "main"

When no explicit base_branch input is provided, the action previously
fell back to a hardcoded "main", which fails on repositories whose
default branch is named differently (e.g. "master", "develop").

This reads repository.default_branch from the GitHub event payload
(populated once in parseGitHubContext) and uses it as the fallback in
all three callsites: agent/index.ts, run.ts, and update-comment-link.ts.
Explicit env/input precedence is preserved; "main" remains only as a
last-resort defensive fallback if the payload somehow lacks the field.

* test: drop unused BASE_BRANCH env handling from default_branch test

agent/index.ts no longer reads process.env.BASE_BRANCH directly (it now
goes through context.inputs.baseBranch which is set on the mock context),
so saving/clearing/restoring that env var in the regression test is dead
code.

test/modes/agent.test.ts

@@ -88,7 +88,7 @@ describe("Agent Mode", () => {
     expect(result.claudeArgs).toBe("--model claude-sonnet-4 --max-turns 10");
     expect(result.claudeArgs).not.toContain("--mcp-config");
 
-    // Verify return structure - should use "main" as fallback when no env vars set
+    // Verify return structure - should fall back to repository.default_branch when no env vars set
     expect(result).toEqual({
       commentId: undefined,
       branchInfo: {
@@ -108,6 +108,60 @@ describe("Agent Mode", () => {
       process.env.GITHUB_REF_NAME = originalRefName;
   });
 
+  test("prepare falls back to repository.default_branch when not 'main'", async () => {
+    const contextWithDevelop = createMockAutomationContext({
+      eventName: "workflow_dispatch",
+      repository: {
+        owner: "test-owner",
+        repo: "test-repo",
+        full_name: "test-owner/test-repo",
+        default_branch: "develop",
+      },
+    });
+
+    // Save and clear env vars that would otherwise override the fallback
+    const originalClaudeBranch = process.env.CLAUDE_BRANCH;
+    const originalHeadRef = process.env.GITHUB_HEAD_REF;
+    const originalRefName = process.env.GITHUB_REF_NAME;
+    delete process.env.CLAUDE_BRANCH;
+    delete process.env.GITHUB_HEAD_REF;
+    delete process.env.GITHUB_REF_NAME;
+
+    const mockOctokit = {
+      rest: {
+        users: {
+          getAuthenticated: mock(() =>
+            Promise.resolve({
+              data: { login: "test-user", id: 12345, type: "User" },
+            }),
+          ),
+          getByUsername: mock(() =>
+            Promise.resolve({
+              data: { login: "test-user", id: 12345, type: "User" },
+            }),
+          ),
+        },
+      },
+    } as any;
+
+    const result = await prepareAgentMode({
+      context: contextWithDevelop,
+      octokit: mockOctokit,
+      githubToken: "test-token",
+    });
+
+    expect(result.branchInfo.baseBranch).toBe("develop");
+    expect(result.branchInfo.currentBranch).toBe("develop");
+
+    // Restore env vars
+    if (originalClaudeBranch !== undefined)
+      process.env.CLAUDE_BRANCH = originalClaudeBranch;
+    if (originalHeadRef !== undefined)
+      process.env.GITHUB_HEAD_REF = originalHeadRef;
+    if (originalRefName !== undefined)
+      process.env.GITHUB_REF_NAME = originalRefName;
+  });
+
   test("prepare rejects bot actors without allowed_bots", async () => {
     const contextWithPrompts = createMockAutomationContext({
       eventName: "workflow_dispatch",