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/mockContext.ts

@@ -36,6 +36,7 @@ const defaultRepository = {
   owner: "test-owner",
   repo: "test-repo",
   full_name: "test-owner/test-repo",
+  default_branch: "main",
 };
 
 type MockContextOverrides = Omit<Partial<ParsedGitHubContext>, "inputs"> & {