docs: address PR #1121 review — respect user preference, drop y/n

- Consent prompt: drop "(y/n)" and add escape valve for users who
  have already declared their worktree preference in global or
  project agent instruction files.
- Directory selection: reorder to put declared user preference
  ahead of observed filesystem state, and reframe the default as
  "if no other guidance available".
- Sandbox fallback: require explicitly informing the user that
  the sandbox blocked creation, not just "report accordingly".
- writing-plans: fully qualify the superpowers:using-git-worktrees
  reference.
- Plan doc: mirror the consent-prompt change.

Step 1a native-tool framing and the helper-scripts suggestion are
still outstanding — the first needs a benchmark re-run before softer
phrasing can be adopted without regressing compliance; the second is
exploratory and will get a thread reply.

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

skills/writing-plans/SKILL.md

@@ -13,7 +13,7 @@ Assume they are a skilled developer, but know almost nothing about our toolset o
 
 **Announce at start:** "I'm using the writing-plans skill to create the implementation plan."
 
-**Context:** If working in an isolated worktree, it should have been created via the using-git-worktrees skill at execution time.
+**Context:** If working in an isolated worktree, it should have been created via the `superpowers:using-git-worktrees` skill at execution time.
 
 **Save plans to:** `docs/superpowers/plans/YYYY-MM-DD-<feature-name>.md`
 - (User preferences for plan location override this default)