Sprint 291: Add HTTP/SSE daemon mode to whetstone_mcp (Steps 1983-1987)

Eliminates the dual-process bug where multiple Claude Code sessions each
spawn their own whetstone_mcp stdio process with split in-memory state.
All sessions now share a single persistent daemon via MCP HTTP+SSE transport.

Changes:
- MCPHttpServer.h: HTTP+SSE transport (cpp-httplib via FetchContent).
  GET /sse streams events per session, POST /message routes JSON-RPC,
  GET /health for status. Thread-safe session map with mutex/cv/queue.
- MCPBridge.h: adds runHttp(port, workspace) method
- mcp_main.cpp: --daemon/--port flags, daemon.pid lockfile, duplicate guard
- CMakeLists.txt: FetchContent for cpp-httplib, step1983-1987 targets
- tools/start-whetstone-daemon.sh: idempotent startup, health-check poll
- 5/5 tests passing (step1987_test)

Note: Documents/.mcp.json switched to SSE transport in working directory
(not tracked in this repo). Daemon must be started before Claude Code sessions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tools/start-whetstone-daemon.sh

@@ -0,0 +1,80 @@
+#!/usr/bin/env bash
+# start-whetstone-daemon.sh
+#
+# Ensures a single whetstone_mcp HTTP+SSE daemon is running on port 7600.
+# Safe to call multiple times — exits immediately if already running.
+#
+# Usage:
+#   ./tools/start-whetstone-daemon.sh
+#   ./tools/start-whetstone-daemon.sh --port 7601
+#   ./tools/start-whetstone-daemon.sh --language python
+#
+# Output: prints daemon PID on stdout.
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+WHETSTONE_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
+BINARY="$WHETSTONE_DIR/editor/build-native/whetstone_mcp"
+WORKSPACE="$WHETSTONE_DIR"
+PORT=7600
+LANGUAGE="cpp"
+LOG="$WHETSTONE_DIR/.whetstone/daemon.log"
+PID_FILE="$WHETSTONE_DIR/.whetstone/daemon.pid"
+
+# Parse optional overrides
+while [[ $# -gt 0 ]]; do
+    case "$1" in
+        --port)     PORT="$2";     shift 2 ;;
+        --language) LANGUAGE="$2"; shift 2 ;;
+        *) echo "Unknown arg: $1" >&2; exit 1 ;;
+    esac
+done
+
+# Check if daemon is already running
+if [[ -f "$PID_FILE" ]]; then
+    EXISTING_PID=$(cat "$PID_FILE" 2>/dev/null || echo "")
+    if [[ -n "$EXISTING_PID" ]] && kill -0 "$EXISTING_PID" 2>/dev/null; then
+        echo "[whetstone-daemon] Already running (PID $EXISTING_PID)" >&2
+        echo "$EXISTING_PID"
+        exit 0
+    fi
+fi
+
+# Also check by port
+if lsof -ti :"$PORT" >/dev/null 2>&1; then
+    echo "[whetstone-daemon] Port $PORT already in use — daemon may be running" >&2
+    lsof -ti :"$PORT"
+    exit 0
+fi
+
+if [[ ! -x "$BINARY" ]]; then
+    echo "[whetstone-daemon] Binary not found: $BINARY" >&2
+    echo "[whetstone-daemon] Build it with: cmake --build editor/build-native --target whetstone_mcp --parallel" >&2
+    exit 1
+fi
+
+mkdir -p "$WHETSTONE_DIR/.whetstone"
+
+# Start daemon in background
+nohup "$BINARY" \
+    --daemon \
+    --port "$PORT" \
+    --workspace "$WORKSPACE" \
+    --language "$LANGUAGE" \
+    > "$LOG" 2>&1 &
+
+DAEMON_PID=$!
+
+# Wait briefly for daemon to write its PID file and start listening
+for i in $(seq 1 20); do
+    sleep 0.25
+    if curl -sf "http://127.0.0.1:$PORT/health" >/dev/null 2>&1; then
+        echo "[whetstone-daemon] Started (PID $DAEMON_PID, port $PORT)" >&2
+        echo "$DAEMON_PID"
+        exit 0
+    fi
+done
+
+echo "[whetstone-daemon] WARNING: daemon started (PID $DAEMON_PID) but /health not yet responding" >&2
+echo "$DAEMON_PID"