docs: clean up pre-existing staleness across the guides

Sweep of accumulated drift unrelated to the CLI reorg:
- Source paths: ~/.castle/code/<name> → /data/repos/<name> (REPOS_DIR), and
  source: code/X → absolute /data/repos/X. registry.md source-resolution table
  and 'how programs get in' section rewritten for the /data/repos layout.
- Old field name component: → program: in all YAML examples.
- Old term 'component' → 'program' throughout design.md and the stack guides.
- Frontend serving: corrected the copy-to-artifacts/content model to serve-in-
  place from the repo (<source>/<dist>), matching Phase 2b; dropped the dead
  _copy_app_static reference and the 'runtime never references the source tree'
  claim (frontends are the deliberate exception).
- Stale endpoints/commands: /components → /deployments; castle create/add →
  castle program create/add in the create.py/add.py docstrings.

CLAUDE.md was already current. cli 24 green; ruff clean.

cli/src/castle_cli/commands/create.py

@@ -1,4 +1,4 @@
-"""castle create - scaffold a new project from templates."""
+"""castle program create — scaffold a new program from templates."""
 
 from __future__ import annotations