plugin.json: add explicit version; reconsider skill name

1. Add an explicit version to plugin.json

There's no version today. When it's omitted, Claude Code versions the plugin by its git commit SHA — fine for git-based marketplace installs, but when the plugin ships as part of the released Swift package there's no git context, so no SHA. Then the Claude Code plugin cache (~/.claude/plugins/cache) can't tell versions apart and autoUpdate has nothing to compare. Set an explicit version at release, matching the package version.

2. Skill name doubles under the plugin namespace

Claude Code renders plugin skills as plugin:skill → here implicits:implicits-usage-guide. One name can't be both clean under that render and self-descriptive when surfaced bare:

• (a) rename → usage-guide → implicits:usage-guide (clean); but bare usage-guide is ambiguous if the skill is ever surfaced outside the implicits: prefix.
• (b) keep implicits-usage-guide; self-contained, but the render stays doubled.

Gate: is the skill ever surfaced without the implicits: plugin prefix? No → (a), yes → (b).

Start by reading plugin.json and the Swift package's release/version metadata, then verify how the skill is surfaced with and without the implicits: prefix. Done means plugin.json carries the release version and the skill name follows the outcome of that check; the issue names no test file or command.