chore: split aixgate into its own repository

Aixgate (the runtime sandbox for AI coding agents) has moved to its own
repository at https://github.com/aixgo-dev/aixgate.

The decision is recorded in ADR 0002, which supersedes ADR 0001's
"keep aixgate in the monorepo" decision. Two of the three
"reconsider when..." triggers ADR 0001 explicitly named are now firing
pre-v0.1: release-cadence divergence and the planned aixgo-cloud
commercial variant. The AIXGATE_PRD itself already disagreed with the
monorepo assumption (§10.2 one-way optional dependency, §11.4 orthogonal
roadmap, §14.6 independent docs surface). Three independent strategic
reviews (CRO, CMO, product-manager) recommended the split unanimously
on 2026-05-03.

Changes:
- Delete docs/AIXGATE_PRD.md (moved to aixgate/docs/PRD.md)
- Add docs/AIXGATE.md as a one-paragraph pointer to the new repo
- Mark ADR 0001 as Superseded by ADR 0002
- Add ADR 0002: Aixgate ships from a separate repository

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: charlesgreen

docs/AIXGATE.md

@@ -0,0 +1,11 @@
+# Aixgate
+
+Aixgate is a runtime sandbox for AI coding agents (Claude Code, Cursor, Aider, OpenAI Codex). It now lives in its own repository:
+
+- **Repository**: <https://github.com/aixgo-dev/aixgate>
+- **PRD**: <https://github.com/aixgo-dev/aixgate/blob/main/docs/PRD.md>
+- **Status**: pre-v0.1, in active development
+
+The decision to move Aixgate out of the aixgo monorepo is recorded in [ADR 0002](adr/0002-aixgate-separate-repo.md) (which supersedes [ADR 0001](adr/0001-aixgate-monorepo.md)).
+
+> aixgo.dev builds agents. Aixgate keeps them in their lane.