Move SPEC.md to development/ and rewrite as a technical reference

The old spec was a 1426-line forward-looking planning doc (phases,
pending decisions, design sketches for work now shipped). Replace it
with a concise 339-line reference describing the implemented system —
architecture, AST, LSP capabilities + executeCommand surface, editor
integration, config schema, keymaps, syntax support, and CI — verified
against the current code. Lives under development/ with the other
developer-only docs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

development/ONBOARDING.md

@@ -70,6 +70,7 @@ nuwiki/
 │
 ├── development/                # Developer-only tooling and docs (not shipped)
 │   ├── ONBOARDING.md           # This file
+│   ├── SPEC.md                 # Technical reference (architecture, LSP, commands)
 │   ├── nuwiki-architecture.html
 │   ├── start-nvim.sh           # Launch Neovim against a generated sample wiki
 │   ├── start-vim.sh            # Launch Vim against a generated sample wiki