Improve plugin source modeling and runtime dedup

[gschier]

Summary

• add plugins.source migration/backfill and enforce unique plugins.directory
• model plugin source as a typed PluginSource enum and expose it in TS bindings
• make plugin upserts directory-aware by reusing existing plugin id for matching directory
• set explicit source on bundled/filesystem/registry plugin write paths
• dedupe runtime plugin loading by metadata name with precedence filesystem > registry > bundled
• filter registry update checks by source == registry
• update settings plugin tabs to use plugin.source instead of directory heuristics
• return resolved plugin set from models_workspace_models so frontend counts match loaded plugins

Validation

• cargo check -p yaak-models -p yaak-plugins -p yaak-cli
• cargo check --manifest-path crates-tauri/yaak-app/Cargo.toml
• npm run lint (existing repo warnings only)

[chatgpt-codex-connector]

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[gschier]

@codex review

[chatgpt-codex-connector]

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