fix(git): add git.base_branch config to replace hardcoded main target

Adds `git.base_branch` config option that controls the target branch
for PRs and merges. When unset, auto-detects from origin/HEAD and
falls back to "main".

This fixes projects using `master`, `develop`, or any other default
branch — previously `/gsd:ship` would create PRs targeting `main`
(which may not exist) and `/gsd:complete-milestone` would try to
checkout `main` and fail.

Changes:
- config.cjs: add git.base_branch to valid config keys
- planning-config.md: document the option with auto-detect behavior
- ship.md: detect base branch at init, use in PR create, branch
  detection, push report, and completion report
- complete-milestone.md: detect base branch, use for squash merge
  and merge-with-history checkout targets
- 1 new test for config-set git.base_branch

Usage:
  gsd-tools config-set git.base_branch master

Or auto-detect (default — reads origin/HEAD):
  git.base_branch: null

Closes #1466

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/config.test.cjs

@@ -242,6 +242,16 @@ describe('config-set command', () => {
     assert.strictEqual(config.workflow.use_worktrees, false);
   });
 
+  test('sets git.base_branch for non-main default branches', () => {
+    writeConfig(tmpDir, {});
+
+    const result = runGsdTools('config-set git.base_branch master', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const config = readConfig(tmpDir);
+    assert.strictEqual(config.git.base_branch, 'master');
+  });
+
   test('errors when no key path provided', () => {
     const result = runGsdTools('config-set', tmpDir);
     assert.strictEqual(result.success, false);