feat(make): add image-debug target for interactive container debugging

[cvetty]

Summary

• Adds make image-debug — builds the image then starts a detached keep-alive container (sleep infinity) named whygraph-debug, bind-mounting the current repo at /workspace
• Adds make image-debug-down — stops and removes the container
• Adds DEBUG_NAME ?= whygraph-debug variable (overridable like IMAGE)

Motivation

The production shim runs each whygraph command as an ephemeral docker run --rm, which makes it impossible to exec in and poke around. This target keeps the container alive so you can open multiple shells, run commands interactively, and inspect /workspace without re-building.

Usage

make image-debug                      # build + start
docker exec -it whygraph-debug bash   # drop into the container
make image-debug-down                 # tear down

The container mirrors the real run environment: same -v $PWD:/workspace -w /workspace --user $(id -u):$(id -g) -e HOME=/tmp flags as the production shim, so .whygraph/ / .codegraph/ ownership and config discovery match a real whygraph invocation.

Test plan

• make image-debug builds successfully and prints the exec hint
• docker exec -it whygraph-debug bash drops into /workspace with the repo mounted
• whygraph version works inside the container
• make image-debug-down removes the container cleanly
• Re-running make image-debug is idempotent (replaces stale container)
• make help shows both new targets