Add Antigravity 2.0 IDE + CLI MCP support

[rhennigan]

Adds two changes for Google's Antigravity 2.0 platform launched at I/O 2026:

1. New "AntigravityCLI" client entry for the terminal agent that replaces Gemini CLI for free/consumer tiers on June 18, 2026. It reads from ~/.gemini/antigravity-cli/mcp_config.json (global) and .agents/mcp_config.json (workspace) -- sibling to but distinct from the existing "Antigravity" IDE entry. Workspace skills also moved from .gemini/skills/ to .agents/skills/.

2. Auto-detection for the Antigravity 2.0 IDE config path. The 2.0 installer migrates pre-2.0 IDE state forward by dropping a ~/.gemini/config/.migrated marker, after which the running IDE reads its MCP config from ~/.gemini/config/mcp_config.json instead of the historical ~/.gemini/antigravity/mcp_config.json. Without detection, a single InstallMCPServer["Antigravity"] could write to a file the migrated IDE ignores, leaving "No MCP Servers" in the Customizations panel even though the file looked correct on disk. The new antigravityInstallLocation helper checks for the marker on each lookup so the right path is picked for both fresh and migrated installs.

Also documents known Antigravity 2.0 failure modes (app restart, installer directory conflict, macOS GUI $PATH inheritance, WSL perms, cloud-sync override) and the post-migration ~/.gemini/ sibling directory layout so users don't mistake migration-artifact mcp_config.json files in antigravity-ide/, antigravity-backup/, or config/ for the active one. Flags the Gemini CLI deprecation in its docs section.

Tests stub FileExistsQ to exercise both branches of antigravityInstallLocation and cover the new AntigravityCLI entry's install location, display name, alias resolution, and project install location.

[Copilot]

Pull request overview

Adds two MCP client integrations tied to Google's "Antigravity 2.0" rollout: a new AntigravityCLI client entry and a runtime detection helper that routes the existing Antigravity entry to either the pre‑2.0 path (~/.gemini/antigravity/mcp_config.json) or the post‑migration path (~/.gemini/config/mcp_config.json) based on a .migrated marker file. Documentation is expanded with a deprecation notice for Gemini CLI, the new CLI entry, and an Antigravity 2.0 troubleshooting / directory‑layout section.

Changes:

• New antigravityInstallLocation[] helper in SupportedClients.wl that conditionally returns the migrated vs. historical Antigravity IDE config path, wired into the existing "Antigravity" entry via RuleDelayed.
• New "AntigravityCLI" client entry with GoogleAntigravityCLI alias, .agents/mcp_config.json project path, and ~/.gemini/antigravity-cli/mcp_config.json global path.
• Tests covering both branches of antigravityInstallLocation (via a Block‑scoped FileExistsQ stub), install/project/display‑name/alias resolution for AntigravityCLI, and updates to the supported‑client count (18 → 19) and key list.
• Documentation updates in docs/mcp-clients.md covering the Gemini CLI deprecation notice, the migration‑aware Antigravity path table, an Antigravity 2.0 troubleshooting section, the post‑migration ~/.gemini/ sibling layout, and a new Antigravity CLI section.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Kernel/SupportedClients.wl	Switches "Antigravity" to a delayed antigravityInstallLocation[] lookup and adds the new "AntigravityCLI" entry plus helper definition.
Tests/InstallMCPServer.wlt	Adds tests for migrated/fresh Antigravity path routing and for AntigravityCLI install/project/display/alias resolution; updates client count and key‑list expectations.
docs/mcp-clients.md	Adds the new client row, Gemini CLI deprecation notice, Antigravity 2.0 migration/troubleshooting/directory sections, and the dedicated Antigravity CLI section.

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