cbox manager default parent-branch policy

Problem

When a cbox manager agent launches a named sandbox (cbox new <name>), the worktree was historically based on main. When the manager is already on a feature branch, the sandbox should inherit that branch as its parent so work stays on the right lineage without requiring an explicit --parent-branch flag every time.

Context

Possible Solutions

Plan

Implementation Progress

• _default_parent_branch() helper returns the caller's current branch.
• cbox new <name> uses current branch as default parent when --parent-branch is omitted.
• --parent-branch <branch> still overrides the default.
• Console prints Using current branch as parent: <branch> when defaulting.
• Canonical docs updated (docs/agent/cbox.md, .cursor/rules/cbox.mdc, libs/cbox/CLAUDE.md, .codex/skills/cbox/SKILL.md).
• Focused unit tests for _default_parent_branch() and the defaulting behavior in cbox new.

• Manager skill docs updated to reflect that --parent-branch is optional when already on the target branch.

• Named sandbox worktrees default to the manager's current branch, not main.

• Explicit --parent-branch flag still overrides the default.
• _default_parent_branch() returns None on detached HEAD or git failure.
• All cbox tests pass (110/110).

Evidence of prior behavior

Before commit 48354c84 ("unify cbox agent guidance and improve sandbox defaults"), _create_worktree() was called without a parent branch when --parent-branch was omitted, causing git branch sandbox/<name> to branch from HEAD of whatever branch the repo happened to be on — but the docs and skill files all said "parent branch: main", reinforcing a workflow where managers always passed --parent-branch main explicitly. The new behavior makes the implicit default match what managers actually want: inherit the current branch.

• Implementation landed in 48354c84, docs in 30310067.
• This task adds tests, updates skill/workflow docs, and creates a formal task record.

Review Feedback

• [ ] Review cleared