fix(build): auto-run api-gen when generated files are missing

[neptunix]

Problem

go build fails after a plain git clone because the generated API files (api/clickupv{2,3}/*.gen.go, internal/apiv{2,3}/operations.gen.go) are gitignored. This produces 14 compile errors for types that only exist in the generated code:

internal/apiv3/chat.go: undefined: GetChatChannelsParams
pkg/cmdutil/status.go: undefined: apiv2.GetSpace
... (14 total)

Any Dockerfile or CI pipeline that does git clone && go build is broken.

Fix

One line added to the build target — if the generated files are absent, run make api-gen first:

build:
	@[ -f api/clickupv3/client.gen.go ] || $(MAKE) api-gen
	go build ...

Verification

git clone https://github.com/triptechtravel/clickup-cli && cd clickup-cli
rm -f api/clickupv2/*.gen.go api/clickupv3/*.gen.go internal/apiv*/operations.gen.go
make build   # triggers api-gen then builds successfully

[isaacrowntree]

Useful fix — solves the real "fresh clone → 14 compile errors" problem with minimal surface area.

Suggestion: apply the same guard to install and test

Both targets have the identical fresh-clone problem. Could either repeat the guard or factor it out:

```makefile
.PHONY: ensure-gen
ensure-gen:
@[ -f api/clickupv3/client.gen.go ] || $(MAKE) api-gen

build: ensure-gen
go build -ldflags "$(LDFLAGS)" -o bin/$(BINARY) ./cmd/clickup

install: ensure-gen
go install -ldflags "$(LDFLAGS)" ./cmd/clickup

test: ensure-gen
go test ./...
```

Other note: this silently adds a network dependency to make build on fresh clones (api-gen curls the OpenAPI specs from developer.clickup.com). Acceptable since the alternative is failing the build outright, but worth a line in the README so airgapped/offline builders aren't surprised.

LGTM otherwise.