[None][infra] Add dev-container entrypoint dispatcher for CLI docker workflow

[jieli-matrix]

Why

The only supported container dev flow today is the VSCode Dev Container (.devcontainer/).
For a CLI-only developer it is a poor fit, and there
is no first-class path for the everyday bug-repro-and-fix loop: pull image → mount TOT +
model data → build (ccache, correct GPU arch) → pytest to repro → branch → incremental
rebuild → pytest to validate.

This PR adds a CLI-only, docker-compose develop-and-repro workflow that sits beside the
VSCode flow (which stays untouched), with a clean up -d → exec loop and no generated env file.

Scope — where this runs, and what it does / doesn't cover

The two modes (rootful / rootless) select for docker daemon. Mapping to real environments:

Environment	Container tech	This workflow
Colossus / server station (interactive dev box)	docker, real root (-u root:root, --privileged)	rootful mode
Locked-down / shared HPC dev box (no root docker)	rootless docker	rootless mode
SLURM cluster (single- and multi-node srun jobs)	enroot/pyxis (--container-remap-root)	not a target — use examples/disaggregated/slurm/

Insights & analysis

• The VSCode flow doesn't transfer to CLI. It depends on make_env.py generating
  .devcontainer/.env (run as VSCode's initializeCommand), and docker-compose.yml's command
  is just while sleep 1000 — a keep-alive with no build/test ergonomics.

• up -d + exec beats run -it. Start a detached, named container once; drive builds/tests
  via exec.

TODO

• .devcontainer/entrypoint.dev.sh — build/test dispatcher (build-full / build-cpp /
  build-python / build-precompiled / test / build-and-test / wait / shell). (first commit)
• docker/compose.dev.yml (base: ipc: host, network_mode: host, init: true, shm_size
  1g/DEV_SHM_SIZE, ulimits.stack=67108864, caps — cluster runtime parity for single-node
  disagg) + docker/compose.dev.rootful.yml (adds user: + ulimit memlock=-1)
• docker/dev wrapper — repo-local -f/--env-file; DEV_ROOTLESS=0|1 selector
• Per-user config templates: docker/compose.dev.override.yml.example,
  docker/dev.user.env.example (git-ignore the real files)
• Default CUDA arch → native in the entrypoint (fix the 90-real default)
• Image-drift stamp (cpp/build/.built_with_image) → warn to build-full --clean on tag change
• Developer-guide README for the CLI bug-repro loop

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