fix(sandbox): use spec-required cleveragents/plan- branch prefix in GitWorktreeSandbox

- Implemented branch naming change in GitWorktreeSandbox.create() from sandbox/<plan_id> to cleveragents/plan-<plan_id>, located in src/cleveragents/infrastructure/sandbox/git_worktree.py
- Updated the create() method docstring to reflect the new branch naming convention
- Updated test fixtures:
  - features/steps/git_worktree_coverage_boost_steps.py: sandbox/test -> cleveragents/plan-test, sandbox/cleanup-test -> cleveragents/plan-cleanup-test, sandbox/branch-delete-fail -> cleveragents/plan-branch-delete-fail
- Updated scenario:
  - features/garbage_collection.feature: sandbox/test-plan -> cleveragents/plan-test-plan
- Design rationale:
  - Adopted the spec-required format: cleveragents/plan-{safe_plan_id} to align with the spec-defined namespace for all CleverAgents-managed git branches
  - Ensures consistent branch naming across code, tests, and features for easier policy enforcement and maintainability

ISSUES CLOSED: #2601

features/garbage_collection.feature

@@ -45,7 +45,7 @@ Feature: CONC3 - Garbage collection and cleanup
     And the cleanup summary should report 1 sandbox removed
 
   Scenario: Git worktree sandbox cleanup removes branch
-    Given a stale git worktree sandbox with branch "sandbox/test-plan"
+    Given a stale git worktree sandbox with branch "cleveragents/plan-test-plan"
     When I purge stale sandboxes
     Then the worktree directory should be removed
     And the cleanup summary should report 1 sandbox removed

features/steps/git_worktree_coverage_boost_steps.py

@@ -207,7 +207,7 @@ def step_gwtcb_commit_modified_diff(ctx: Context) -> None:
     # Set sandbox to ACTIVE with valid worktree/branch
     ctx.gwtcb_sandbox._status = SandboxStatus.ACTIVE
     ctx.gwtcb_sandbox._worktree_path = repo_dir
-    ctx.gwtcb_sandbox._branch_name = "sandbox/test"
+    ctx.gwtcb_sandbox._branch_name = "cleveragents/plan-test"
     ctx.gwtcb_sandbox._base_commit = "abc123"
     ctx.gwtcb_sandbox._original_branch = "main"
 
@@ -284,7 +284,7 @@ def step_gwtcb_active_ready_commit(ctx: Context) -> None:
 
     ctx.gwtcb_sandbox._status = SandboxStatus.ACTIVE
     ctx.gwtcb_sandbox._worktree_path = repo_dir
-    ctx.gwtcb_sandbox._branch_name = "sandbox/test"
+    ctx.gwtcb_sandbox._branch_name = "cleveragents/plan-test"
     ctx.gwtcb_sandbox._base_commit = "abc123"
     ctx.gwtcb_sandbox._original_branch = "main"
 
@@ -436,7 +436,7 @@ def step_gwtcb_cleanup_worktree_exists(ctx: Context) -> None:
     worktree_dir = tempfile.mkdtemp(prefix="gwtcb-worktree-")
     ctx.gwtcb_sandbox._status = SandboxStatus.ACTIVE
     ctx.gwtcb_sandbox._worktree_path = worktree_dir
-    ctx.gwtcb_sandbox._branch_name = "sandbox/cleanup-test"
+    ctx.gwtcb_sandbox._branch_name = "cleveragents/plan-cleanup-test"
 
 
 @given("gwtcb _run_git is mocked to fail on worktree remove")
@@ -488,7 +488,7 @@ def step_gwtcb_cleanup_no_worktree_dir(ctx: Context) -> None:
     # so the worktree removal block is skipped
     ctx.gwtcb_sandbox._status = SandboxStatus.ACTIVE
     ctx.gwtcb_sandbox._worktree_path = "/tmp/nonexistent-gwtcb-worktree"
-    ctx.gwtcb_sandbox._branch_name = "sandbox/branch-delete-fail"
+    ctx.gwtcb_sandbox._branch_name = "cleveragents/plan-branch-delete-fail"
 
 
 @given("gwtcb _run_git is mocked to fail on branch delete")

src/cleveragents/infrastructure/sandbox/git_worktree.py

@@ -168,7 +168,7 @@ class GitWorktreeSandbox:
     def create(self, plan_id: str) -> SandboxContext:
         """Create a git worktree for isolated modifications.
 
-        Creates a new branch ``sandbox/<plan_id>`` and a worktree at a
+        Creates a new branch ``cleveragents/plan-<plan_id>`` and a worktree at a
         temporary directory.  The worktree starts from the current HEAD
         of the repository.
 
@@ -226,7 +226,7 @@ class GitWorktreeSandbox:
 
             # Create a sanitised branch name
             safe_plan_id = _sanitise_branch_name(plan_id)
-            self._branch_name = f"sandbox/{safe_plan_id}"
+            self._branch_name = f"cleveragents/plan-{safe_plan_id}"
 
             # Create a temporary directory for the worktree
             self._worktree_path = tempfile.mkdtemp(prefix=f"ca-sandbox-{safe_plan_id}-")