docs: add project planning and implementation documentation

Add PLAN.md with full project specification and @PLAN.md with
parallel build plan for multi-agent implementation. Also add
IMPLEMENTATION_SUMMARY.md documenting Tasks W, X, Y, Z completion
(rollup drill-down, theme toggle, P0 alerts).

Makefile

@@ -0,0 +1,44 @@
+.PHONY: help tidy fmt vet test build run export clean
+
+GO ?= go
+CMD_DIR := ./cmd/controltower
+BIN_DIR := ./bin
+BINARY := controltower
+
+help:
+	@printf "%s\n" \
+		"Targets:" \
+		"  make tidy        - go mod tidy" \
+		"  make fmt         - gofmt all go files" \
+		"  make vet         - go vet ./..." \
+		"  make test        - go test ./..." \
+		"  make build       - build binary to ./bin/controltower" \
+		"  make run         - run TUI (needs a real TTY)" \
+		"  make export PATH=/tmp/issues.json - export snapshot and exit" \
+		"  make clean       - remove ./bin"
+
+tidy:
+	$(GO) mod tidy
+
+fmt:
+	$(GO)fmt -w .
+
+vet:
+	$(GO) vet ./...
+
+test:
+	$(GO) test ./... -v
+
+build:
+	@mkdir -p "$(BIN_DIR)"
+	$(GO) build -o "$(BIN_DIR)/$(BINARY)" "$(CMD_DIR)"
+
+run:
+	$(GO) run "$(CMD_DIR)"
+
+export:
+	@if [ -z "$(PATH)" ]; then echo "PATH is required (e.g. make export PATH=/tmp/issues.json)"; exit 2; fi
+	$(GO) run "$(CMD_DIR)" --export "$(PATH)"
+
+clean:
+	rm -rf "$(BIN_DIR)"