feat: Add antigravity support

[Pringled]

This PR adds support for Antigravity. It uses a bunch of things from Gemini CLI which is nice. I also ordered the installer to show detected ones first.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/semble/installer/agents.py	100.00% <ø> (ø)
src/semble/installer/installer.py	100.00% <ø> (ø)

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[greptile-apps]

Confidence Score: 3/5

Merging as-is will silently break Gemini CLI instructions whenever a user uninstalls Antigravity (or vice versa), because both agents write to the same GEMINI.md file.

The shared instructions_path creates a real, reproducible data-loss scenario: one uninstall removes a configuration block that the other agent still needs. The rest of the change (sorting, MCP config, subagent, docs) is straightforward and correct.

src/semble/installer/agents.py — the Antigravity instructions_path needs to be distinct from the Gemini CLI entry.

_{Reviews (1): Last reviewed commit: "Update docs" | Re-trigger Greptile}