feat: onboarding script, full lh flag reference, expanded README

- scripts/onboard.mjs: bootstraps a target repo onto the harness — copies
  the behaviour layer (.github/agents, skills, instructions, prompts,
  mcp.json, copilot-instructions.md, AGENTS.md), npm links the `lh` CLI,
  runs `lh init` + `lh doctor`, prints next steps. Idempotent: identical
  files are skipped, differing files require --force, re-running `lh init`
  on an initialized repo warns instead of failing. Supports --dry-run and
  --no-npm-link for CI/sandboxed use.
- tests/onboard.test.mjs: 5 new e2e tests (dry-run, full run, idempotent
  rerun, missing target dir, conflict + --force).
- README: new "Onboarding a new project" section, full lh flag reference
  for every subcommand (previously only one-line summaries), Development
  section mentions the onboarding script.

Live-tested against ~/Sources/ralph-runtime (a real, not-yet-git-tracked
project): git init, .github/ copied, `npm link` succeeded, `lh init`
ran, `lh doctor` correctly flagged its one real gap (no verify command
configured) rather than a false pass.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

package.json

@@ -27,7 +27,8 @@
   "scripts": {
     "validate": "node scripts/validate.mjs",
     "test": "node --test tests/",
-    "lh": "node src/cli.mjs"
+    "lh": "node src/cli.mjs",
+    "onboard": "node scripts/onboard.mjs"
   },
   "dependencies": {
     "web-tree-sitter": "^0.25.10"