[codex] add worktree-first specify command

[dango85]

Summary

Adds an explicit worktree-first specify command for the workflow discussed in issue #4: create or reuse the feature worktree before writing any spec artifacts, then continue the normal Spec Kit flow from that worktree root.

Why

The existing after_specify hook creates a worktree only after /speckit.specify has already written artifacts in the primary checkout. A raw before_specify hook can create a worktree, but it does not guarantee Spec Kit will switch the active project root for the rest of the command. This PR documents and exposes the predictable command-level workflow instead.

Changes

• Add /speckit.worktrees.specify command instructions.
• Register the new command in extension.yml.
• Document the worktree-first VS Code flow in the README.
• Add an Unreleased changelog entry.

Existing PR check

Checked #7 first: tests are green and GitHub reports it as mergeable. It is still marked BLOCKED by GitHub, which appears to be repository policy/review state rather than a code conflict.

Validation

• bash tests/test-create-worktree.sh
• bash tests/test-post-install.sh
• ruby -e 'require "yaml"; YAML.load_file("extension.yml")'
• git diff --check

Closes #4