feat: add --high-mem for high-RAM machine shape selection

[zoutei]

Summary

Adds a --high-mem flag to request high-RAM machine shapes when allocating runtimes. This matches the Colab web UI and colab-vscode, which send shape=hm on /tun/m/assign.

• colab new, colab run, and colab ssh (auto-create only) accept --high-mem
• colab sessions and colab status show machine shape (Standard / High-RAM)
• L4 and TPU accelerators ignore the flag (single-shape only), with a stderr notice

Partially addresses #47 (section 1 only).

Changes

• Pass shape=hm through the assign client for supported accelerators
• Add --high-mem to new, run, and ssh
• Persist machine_shape in session state
• Update README and design docs

Testing

Tested manually on Colab Pro with uv run pytest tests/ (341 passed).

Allocation and shape

• colab new --high-mem → High-RAM CPU, ~54.8 GB RAM
• colab new (no flag) → Standard CPU, ~13.6 GB RAM
• colab new --gpu T4 --high-mem → High-RAM T4, ~54.8 GB RAM
• colab new --gpu L4 --high-mem → ignore message, Standard shape
• colab new --tpu v5e1 --high-mem → ignore message

Run and display

• colab run --high-mem and colab run --gpu T4 --high-mem --keep → High-RAM in sessions/status
• colab ssh -s <existing> --high-mem → warning that flag is ignored on existing sessions

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[sethtroisi]

Looks good. merging and I'll test and include in the next release.