feat(setup): add antigravity agent support to `engram setup`

[alexandervazquez98]

📋 Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

🔍 Problem Description

Running engram setup antigravity currently returns:
unknown agent: "antigravity" (supported: opencode, claude-code, gemini-cli, codex)
Antigravity IDE is already listed in the README architecture diagram and in docs/AGENT-SETUP.md
as a supported consumer of engram via MCP — but it requires fully manual JSON configuration.
There is no engram setup antigravity automation.
Users installing the Gentleman AI stack via gentle-ai (https://github.com/Gentleman-Programming/gentle-ai)
hit this warning during the install pipeline when selecting Antigravity as their agent.
gentle-ai already passes "antigravity" to engram setup but the command rejects it.

💡 Proposed Solution

Add engram setup antigravity support following the same pattern as the existing 4 agents.
What it should do:

1. Write the MCP server entry to ~/.gemini/antigravity/mcp_config.json:
   {
   "mcpServers": {
   "engram": {
   "command": "engram",
   "args": ["mcp", "--tools=agent"]
   }
   }
   }
2. Write the Memory Protocol to ~/.gemini/GEMINI.md (same path used by Antigravity
   as its global system prompt / rules file)
3. Print post-install guidance to restart Antigravity and verify the MCP config
   Implementation touch points (based on existing code):

• internal/setup/setup.go — add to SupportedAgents() + Install() switch + installAntigravity()
• cmd/engram/main.go — add antigravity case to printPostInstall() and printUsage()
• internal/setup/setup_test.go — idempotency test for installAntigravity()

📦 Affected Area

CLI (commands, flags)

🔄 Alternatives Considered

The current workaround is manual JSON config as documented in docs/AGENT-SETUP.md.
This works but breaks the gentle-ai automated install pipeline and requires users
to configure engram manually after installation.

📎 Additional Context

No response