chore: add install scripts, doctor command, and AGENTS.md

- Add one-line installer (scripts/install.sh) with platform detection
- Add doctor command (scripts/commands/doctor.sh) for environment diagnostics
- Add shared libraries: dependencies, docker, platform, validation
- Update README with quick-start installer instructions
- Add AGENTS.md with codebase patterns for AI agent context

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

AGENTS.md

@@ -0,0 +1,37 @@
+# Mosaic Stack — Agent Guidelines
+
+> **Any AI model, coding assistant, or framework working in this codebase MUST read and follow `CLAUDE.md` in the project root.**
+
+`CLAUDE.md` is the authoritative source for:
+
+- Technology stack and versions
+- TypeScript strict mode requirements
+- ESLint Quality Rails (error-level enforcement)
+- Prettier formatting rules
+- Testing requirements (85% coverage, TDD)
+- API conventions and database patterns
+- Commit format and branch strategy
+- PDA-friendly design principles
+
+## Quick Rules (Read CLAUDE.md for Details)
+
+- **No `any` types** — use `unknown`, generics, or proper types
+- **Explicit return types** on all functions
+- **Type-only imports** — `import type { Foo }` for types
+- **Double quotes**, semicolons, 2-space indent, 100 char width
+- **`??` not `||`** for defaults, **`?.`** not `&&` chains
+- **All promises** must be awaited or returned
+- **85% test coverage** minimum, tests before implementation
+
+## Updating Conventions
+
+If you discover new patterns, gotchas, or conventions while working in this codebase, **update `CLAUDE.md`** — not this file. This file exists solely to redirect agents that look for `AGENTS.md` to the canonical source.
+
+## Per-App Context
+
+Each app directory has its own `AGENTS.md` for app-specific patterns:
+
+- `apps/api/AGENTS.md`
+- `apps/web/AGENTS.md`
+- `apps/coordinator/AGENTS.md`
+- `apps/orchestrator/AGENTS.md`