Add interactive tree editor island (Phase 4) + MCP server fixes

Phase 4: defisland ~sx-tools/tree-editor — interactive tree viewer
embedded in the SX Tools page. Features:
- Textarea with :bind for SX source input
- Parse button to re-parse on demand
- Tree view: annotated tree with path labels, clickable nodes
- Context view: enclosing chain from root to selected node
- Validate view: structural integrity checks (catches missing body etc.)

MCP server fixes: added ident-start?, ident-char?, make-keyword,
escape-string, sx-expr-source — needed by parser.sx when loaded
into the MCP evaluator.

Also: .mcp.json for Claude Code MCP server config, CLAUDE.md protocol
for structural .sx file editing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.mcp.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "sx-tree": {
+      "type": "stdio",
+      "command": "./hosts/ocaml/_build/default/bin/mcp_tree.exe"
+    }
+  }
+}