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.

src/entrypoints/run.ts

@@ -229,8 +229,8 @@ async function run() {
     // Restore them from the base branch before the CLI reads them.
     //
     // We read pull_request.base.ref from the payload directly because agent
-    // mode's branchInfo.baseBranch defaults to "main" rather than the PR's
-    // actual target (agent/index.ts). For issue_comment on a PR the payload
+    // mode's branchInfo.baseBranch defaults to the repo's default branch rather
+    // than the PR's actual target (agent/index.ts). For issue_comment on a PR the payload
     // lacks base.ref, so we fall back to the mode-provided value — tag mode
     // fetches it from GraphQL; agent mode on issue_comment is an edge case
     // that at worst restores from the wrong trusted branch (still secure).
@@ -312,7 +312,7 @@ async function run() {
           commentId,
           githubToken,
           claudeBranch,
-          baseBranch: baseBranch || "main",
+          baseBranch: baseBranch || context.repository.default_branch || "main",
           triggerUsername: context.actor,
           context,
           octokit,

src/entrypoints/update-comment-link.ts

@@ -253,7 +253,8 @@ async function run() {
       commentId: parseInt(process.env.CLAUDE_COMMENT_ID!),
       githubToken,
       claudeBranch: process.env.CLAUDE_BRANCH,
-      baseBranch: process.env.BASE_BRANCH || "main",
+      baseBranch:
+        process.env.BASE_BRANCH || context.repository.default_branch || "main",
       triggerUsername: process.env.TRIGGER_USERNAME,
       context,
       octokit,

src/github/context.ts

@@ -79,6 +79,7 @@ type BaseContext = {
     owner: string;
     repo: string;
     full_name: string;
+    default_branch?: string;
   };
   actor: string;
   inputs: {
@@ -140,6 +141,7 @@ export function parseGitHubContext(): GitHubContext {
       owner: context.repo.owner,
       repo: context.repo.repo,
       full_name: `${context.repo.owner}/${context.repo.repo}`,
+      default_branch: context.payload.repository?.default_branch,
     },
     actor: context.actor,
     inputs: {

src/modes/agent/index.ts

@@ -85,15 +85,15 @@ export async function prepareAgentMode({
 
   // Check for branch info from environment variables (useful for auto-fix workflows)
   const claudeBranch = process.env.CLAUDE_BRANCH || undefined;
-  const baseBranch =
-    process.env.BASE_BRANCH || context.inputs.baseBranch || "main";
+  const defaultBranch = context.repository.default_branch || "main";
+  const baseBranch = context.inputs.baseBranch || defaultBranch;
 
   // Detect current branch from GitHub environment
   const currentBranch =
     claudeBranch ||
     process.env.GITHUB_HEAD_REF ||
     process.env.GITHUB_REF_NAME ||
-    "main";
+    defaultBranch;
 
   // Get our GitHub MCP servers config
   const ourMcpConfig = await prepareMcpConfig({