Fix validation of shared source repositories

[steve-calvert-glean]

Summary

• have each target validator report the marketplace files and plugin directories it owns
• scan only those native output paths for surviving partial references
• ignore authored shared/**, workflow examples, dependencies, and other unrelated files that coexist with root-level target output
• preserve detection of stale or unmanaged partial tags inside emitted plugins

Why

Pluginpack 0.11 recommends repositories where authored shared/** sources and generated marketplaces coexist under one root. pluginpack validate --dir <root> currently walks the entire root, so valid authored partials and even Pluginpack’s own node_modules can fail validation.

Validation

• regression test reproduces source/workflow/dependency false positives in the recommended single-repo layout
• existing stale-output partial test remains green
• direct Claude, Cursor, and Codex validation passes against glean-cookbook without its staging wrapper
• mise exec -- npm run test:all (202 passed, 1 skipped)