fix(#105): skip strategy-branch auto-switch when use_worktrees=false (#150)

* fix(#105): skip strategy branch auto-switch when use_worktrees is false

When workflow.use_worktrees is false, the primary checkout is shared or
pinned to a base branch. Auto-switching HEAD in that mode silently moves
the shared checkout and allows concurrent commits to land on the wrong
branch. ensureStrategyBranch now returns ok:true with an explicit skip
reason instead of calling git checkout in this configuration.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test(#105): replace source-grep tests with behavioral coverage

Remove the root tests/bug-105-*.test.cjs file which used source-text
structural assertions (reading commit.ts as a string) — tests that pass
even when the runtime behavior is broken. Replace with a Vitest test at
sdk/src/query/commit.bug-105.test.ts that invokes ensureStrategyBranch
directly with use_worktrees:false and asserts the guard fires before any
git invocation, and that the guard does NOT fire when use_worktrees is
true or absent.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* chore(#105): add changeset fragment

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

* fix(#105): address review — type WorkflowConfig.use_worktrees, accept "false" string, stub git in tests

- Add `use_worktrees?: boolean | string` to WorkflowConfig interface with doc comment
- Remove `(config.workflow as unknown as Record<string, unknown>)` double cast; use typed `config.workflow?.use_worktrees`
- Accept string `"false"` alongside boolean `false` via `isExplicitlyFalse` guard (YAML/JSON parser resilience)
- Add `vi.mock('node:child_process')` stub in commit.bug-105.test.ts; assert no-call on skip-path tests
- Add new test case for string `"false"` coercion

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

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

sdk/src/query/commit.ts

@@ -233,6 +233,22 @@ export async function ensureStrategyBranch(
     return { ok: true, reason: 'strategy-skipped: config load failed' };
   }
 
+  // Issue #105: when use_worktrees is false the primary checkout is shared or
+  // pinned (concurrent sessions / deliberate base-branch lock).  Auto-switching
+  // HEAD in that mode silently moves the shared checkout and causes commits from
+  // parallel sessions to land on the wrong branch.  Skip the switch entirely;
+  // the caller commits on whatever branch the primary is currently on.
+  // String "false" is tolerated for resilience against YAML/JSON parsers that
+  // leave boolean-like fields as strings.
+  const useWorktrees = config.workflow?.use_worktrees;
+  const isExplicitlyFalse = useWorktrees === false || useWorktrees === 'false';
+  if (isExplicitlyFalse) {
+    return {
+      ok: true,
+      reason: 'strategy-skipped: use_worktrees is false — shared/pinned primary checkout; branch auto-switch suppressed (#105)',
+    };
+  }
+
   const strategy = config.git.branching_strategy;
   if (!strategy || strategy === 'none') return { ok: true };