feat(codex): publish repository plugin marketplace

[tulayha]

Summary

• make plugins/vidxp the single editable plugin and skill source, exposed through the repository marketplace at .agents/plugins/marketplace.json
• add a VidXP install/bootstrap skill plus the product logo and composer icon
• make signed beta/stable Desktop builds install the Git marketplace from main/release, while development and PR builds retain a local marketplace
• register the selected runtime's absolute MCP command through codex mcp add and migrate obsolete vidxp-local registrations after a successful Git install
• copy the canonical plugin into Python distributions only at build time and update release/version, CI scope, tests, and documentation contracts

Why

The previous layout kept duplicate editable skill trees and embedded a generic MCP command in the plugin. That could not represent Desktop's private runtime path and made Git marketplace upgrades impossible. The new structure keeps the upgradeable plugin in Git, separates the portable skill bundle from machine-specific MCP registration, and preserves a local path for unreleased builds.

Validation

• official plugin validator: passed
• official quick skill validator for all three skills: passed
• real Codex CLI local marketplace install and absolute MCP registration: passed in an isolated CODEX_HOME
• real Codex CLI Git marketplace sparse clone/install from this remote branch: passed with sourceType: git
• uv run --no-sync ruff check .: passed
• focused Python, packaging, and CI-scope tests: 36 passed
• npm --prefix desktop run check: 38 frontend tests passed; typecheck, lint, and production build passed
• cargo fmt --manifest-path desktop/src-tauri/Cargo.toml --check: passed
• cargo check --locked --manifest-path desktop/src-tauri/Cargo.toml: passed for local and VIDXP_PLUGIN_MARKETPLACE_REF=main builds
• cargo test --locked --manifest-path desktop/src-tauri/Cargo.toml: 79 passed

The complete pytest -q run exceeded a five-minute local command limit without reporting a test failure; all tests covering the changed boundaries passed separately.