fix(cli): accept source prefixes in the wizard package prompt

[mrrobertkent]

Problem

The interactive wizard rejects the source prefixes documented for skilld add, with a stack trace rather than a message:

◇  Enter package names (space or comma-separated)
│  npm:unlighthouse
│
◑  npm:unlighthouse: npm registryError: Invalid package name: npm:unlighthouse
    at getCacheDir (core/prepare.ts)
    at resolvePkgDir
    at getShippedSkills
    at handleShippedSkills
    at npmResolver

■  Canceled

add normalises its inputs:

// src/commands/sync/add.ts
const items = rawInputs.map(parseSkillInput)

The wizard does not:

// src/cli.ts
selected = input.split(COMMA_OR_WHITESPACE_RE).map(s => s.trim()).filter(Boolean)

npm:unlighthouse travels as a literal package name until getCacheDir rejects it, and the throw ends the run.

Change

Both manual-entry prompts normalise through parseSkillInput. npm: prefixed and bare names resolve; other sources report where they belong:

gh:owner/repo is not an npm package. Install it with `skilld add gh:owner/repo`.

resolvePkgDir no longer propagates the validation error. It probes for an optional cache hit, so a name getCacheDir will not accept is a miss. Traversal is still rejected; it just no longer aborts a sync from a read path. An empty name also returns null rather than resolving to <cwd>/node_modules, which join produces for an empty segment.

Coverage

test/unit/pkg-dir-probe.test.ts covers resolvePkgDir returning null for npm:vue, gh:owner/repo, ../escape, and an empty name, and getShippedSkills reporting none for the same.

Docs

README notes that the documented prefixes work in the wizard prompt, and the prompt placeholder shows one.

[harlan-zw]

awesome, thanks!