#2863 feat(config): add YAML startup configuration parity

Summary

• add automatic config.yaml discovery from the working directory, Kandev home, and /etc/kandev, with environment overrides and source provenance
• wire stable startup settings through the launcher, backend, managed agentctl processes, executors, and message-queue settings API
• document the YAML contract, secret handling, exclusions, and service behavior; add tests and desktop/mobile coverage

Verification

• go test ./... (10,134 tests, 155 packages)
• golangci-lint run ./... --new-from-rev=HEAD --timeout=5m
• pnpm --filter @kandev/web lint
• pnpm run typecheck
• pnpm run i18n:check
• node scripts/validate-public-docs.mjs
• desktop message-queue E2E: 6 passed
• mobile message-queue E2E: 2 passed