feat: add Factory AI (Droid) integration

[josephy02]

Factory AI (Droid) integration

Adds first-class support for Factory AI (Droid), revisiting the goal of #195 against current main.

Two surfaces, mirroring how Codex is already supported:

1. gitnexus setup -c droid — registers Droid in the editor-targets.ts abstraction (added since #195), so setup writes the MCP server to ~/.factory/mcp.json and installs skills to ~/.factory/skills/ from the single canonical skills/ source. Because uninstall.ts is target-driven, removal is covered automatically (verified by the setup↔uninstall round-trip test).

2. gitnexus-factory-plugin/ — a standalone plugin for droid plugin install: .factory-plugin/plugin.json, root mcp.json, and a self-contained PostToolUse search-augment hook (Grep|Glob|Execute, ${DROID_PLUGIN_ROOT}).

Addressing the review concerns from #195

#195 concern	Resolution
Skills duplication (7 copied skills)	No copies — skills install from the canonical skills/ root via setup; the plugin ships none
Regex \brg$ didn't match full paths	Uses \brg\b|\bgrep\b
Hook timeout mismatch	Outer 10s / inner 8s aligned
jq dependency	Hook is pure Node, no jq
Staleness / needs rebase	Built against current main

How to verify

cd gitnexus
npx vitest run test/unit/setup.test.ts test/unit/setup-selection.test.ts \
  test/integration/setup-uninstall-roundtrip.test.ts test/unit/uninstall.test.ts
# manual: mkdir -p ~/.factory && gitnexus setup -c droid
#   → writes ~/.factory/mcp.json + ~/.factory/skills/, then `gitnexus uninstall --force` removes them

• New setupDroid unit tests + droid added to the setup↔uninstall round-trip drift test.
• setup-selection valid-values assertion updated.
• Affected suites pass (66 + 34 green); tsc --noEmit clean on the changed files.

Risk / rollback

Additive — a new EditorId and new setup functions gated on ~/.factory/ existing; no existing editor path changes. uninstall fully reverses it. Rollback = revert the commits.

Factory config paths and schemas confirmed from the Factory MCP, skills, and plugins docs.

[vercel]

@josephy02 is attempting to deploy a commit to the NexusCore Team on Vercel.

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