v3: existing-project onboarding

[itseffi]

Summary

• adds productize onboard existing with deterministic preflight, apply, setup, registration, dry-run, and versioned reporting
• adds shared project-root discovery and modular Go, Node, Python, Rust, Compose, CI, documentation, ADR, and workflow-memory inventory
• safely generates the five canonical project-knowledge documents with limits, diagnostics, attribution, protection, and idempotent refreshes
• propagates inventory and diagnostics through sync, archive, daemon, API, OpenAPI, documentation, bundled skills, and operator guidance

Verification

• PATH=/Users/effi/go/bin:$PATH make verify
• golangci-lint: 0 issues
• 3,765 race-enabled tests passed; 3 intentional skips
• binary build passed
• black-box dry-run, apply, and idempotent rerun passed on a mixed-language fixture