chore: add repo-level .mcp.json for leankg, codebase-memory-mcp, playwright

These were only registered in the user's global ~/.claude.json (with
machine-specific absolute paths), so they weren't available to anyone
else working in this repo. Added a repo-root .mcp.json using bare
command names (leankg, codebase-memory-mcp) so it's portable across
machines once those binaries are on PATH, plus the official Playwright
MCP server via npx (no install needed) — closing the "no separate
Playwright MCP server configured" gap noted in
docs/research/smart-tablet-screen-stack.md §5.

Also gitignore .leankg/ (leankg's local runtime pid/index state,
regenerated on start, not source).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Tu1c13iVNdj1iWL633yazG

.mcp.json

@@ -0,0 +1,15 @@
+{
+  "mcpServers": {
+    "leankg": {
+      "command": "leankg",
+      "args": ["mcp-stdio", "--watch"]
+    },
+    "codebase-memory-mcp": {
+      "command": "codebase-memory-mcp"
+    },
+    "playwright": {
+      "command": "npx",
+      "args": ["-y", "@playwright/mcp@latest"]
+    }
+  }
+}