feat(zcode): add full ZCode install-target support across all surfaces

[MoriDanWork]

Added full ZCode (z.ai desktop IDE) install-target support across all surfaces.

This PR closes the integration gap left after the initial ZCode tool mode introduction. The core installer function, single-model launcher, installed-models scanner, CLI flag, and Web Install Endpoints wizard were all missing — making ZCode unusable as an install target outside of the core endpoint-installer module.

Crucially, this change makes ZCode selectable in the Web Install Endpoints wizard, enabling users to install provider catalogs into ZCode from the browser.

Related Issues

• Completes ZCode integration (initial work in ecf740c)

Changes

1. Core Installer (src/core/endpoint-installer.js)

• Added installIntoZCode — writes provider + models into ~/.zcode/v2/config.json and bots-model-cache.v2.json
• Supports scope='all' (full sync) and scope='selected' (merge mode)
• Uses deterministic fcm-{providerKey} namespace — no duplicate entries
• Added dispatch in installProviderEndpoints for zcode canonical mode

2. Single-Model Launcher (src/core/tool-launchers.js)

• Added writeZCodeConfig for the --zcode launch flow

3. Installed Models Scanner (src/core/installed-models-manager.js)

• Added ZCode config paths so scanAllToolConfigs() discovers ZCode models

4. CLI / TUI (src/core/utils.js, src/tui/app.js, src/tui/key-handler.js)

• Added --zcode flag parsing
• Excluded zcode from shouldCheckMissingTool (desktop app, no binary)

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