Adapt worktree lifecycle guidance from Superpowers

[codeforester]

Problem

Base already uses issue-backed branches and sibling worktrees, but the obra/superpowers worktree and branch-finishing skills contain useful guardrails that could make Base's existing workflow harder to misuse.

Reference:

• https://github.com/obra/superpowers/blob/main/skills/using-git-worktrees/SKILL.md
• https://github.com/obra/superpowers/blob/main/skills/finishing-a-development-branch/SKILL.md

Desired Outcome

Adapt the useful worktree lifecycle ideas into Base's existing basectl gh, AGENTS.md, skills.md, and GitHub workflow guidance without changing Base's established worktree directory convention.

Scope

• Document how to detect whether the agent is already in a linked worktree before creating another one.
• Preserve Base's preferred worktree location: ~/work/base-worktrees/<slug>.
• Clarify baseline validation before implementation begins.
• Clarify finish behavior for PR creation, merge cleanup, branch deletion, and remote branch pruning.
• Clarify that PR worktrees should remain available while review feedback is pending.

Non-Goals

• Do not switch Base to .worktrees/.
• Do not replace basectl gh with raw git or raw gh where Base already supports the operation.
• Do not add destructive cleanup behavior without explicit confirmation or provenance checks.

Acceptance Criteria

• Base workflow docs mention existing-worktree detection and sibling worktree conventions.
• Base finish guidance distinguishes PR creation from post-merge cleanup.
• Cleanup guidance avoids removing worktrees that Base did not create or cannot prove are safe to remove.
• Any examples continue to branch from origin/master.

Validation

• Run git diff --check.
• If basectl gh behavior changes, run the relevant command tests.

Demo Impact

None unless the workflow change affects demo-maintenance guidance.