docs: add canonical source onboarding

[br4vesirrobin]

Summary

• replace the recovered-project README with a source-backed onboarding path
• document provider setup, safety boundaries, managed state, updates, and troubleshooting
• distinguish wired behavior from intended or incomplete surfaces

Validation

• npm ci --ignore-scripts
• npx vitest run --coverage
• npx --yes archgate@latest check
• git diff --check
• independent architect and code-review passes on the staged diff

Local environment gaps

• local Elixir testing is blocked because the installed Hex archive was compiled for a different OTP version
• mix format --check-formatted also reports pre-existing formatting drift in six unchanged source/test files
• hosted CI is the fail-closed Elixir validation gate for this README-only PR