fix(ci): use v9 branch config for python-semantic-release (#4)

Replace `branch = "main"` (v7/v8 key) with `[tool.semantic_release.branches.main]`
table (v9 key). The old key is silently ignored by v9, causing releases to never
trigger on the main branch.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: abrichr

pyproject.toml

@@ -68,9 +68,11 @@ target-version = "py310"
 
 [tool.semantic_release]
 version_toml = ["pyproject.toml:project.version"]
-branch = "main"
 commit_message = "chore: release {version}"
 
+[tool.semantic_release.branches.main]
+match = "main"
+
 [tool.semantic_release.commit_parser_options]
 allowed_tags = ["build", "chore", "ci", "docs", "feat", "fix", "perf", "refactor", "style", "test"]
 minor_tags = ["feat"]