chore: speed up Codespaces, remove Claude Code, simplify devcontainer

[idvoretskyi]

Summary

• Drop heavy features: removes docker-in-docker, sshd, and git devcontainer features — the largest contributors to slow Codespaces cold-starts. Docker and SSH remain accessible on Codespaces via host-level tooling.
• Disable zsh/Oh My Zsh in common-utils to reduce feature install time; default shell is now bash.
• Slim Dockerfile: removes build-essential, curl, wget, jq from the apt layer (already provided by the base image and common-utils feature) and drops a dead .ssh directory block.
• Cache OpenCode via prebuilds: moves npm install -g opencode-ai from postCreateCommand to updateContentCommand, so it is baked into Codespaces prebuilds and not re-executed on every container start.
• Remove Claude Code: drops anthropic.claude-code VS Code extension, @anthropic-ai/claude-code npm install, and all related smoke-test checks.
• Remove Docker extension: drops ms-azuretools.vscode-docker (consistent with removing Docker-in-Docker).
• Trim CI: removes claude/docker smoke-test steps, drops unnecessary fetch-depth: 0 from the test job.
• README: removes Claude Code and Docker references, adds a "Speeding Up Codespaces with Prebuilds" setup guide, updates startup-time estimates to 45–75 s (no prebuild) / 10–25 s (with prebuild).

Expected startup time improvement

Scenario	Before	After
Cold-start, no prebuild	~2–3 min	~45–75 s
Cold-start, with prebuild	n/a	~10–25 s
Container restart	~30–60 s	~5–10 s

Notes

• Ubuntu 26.04 base image is not yet published by Microsoft; version bump deferred until mcr.microsoft.com/devcontainers/base:ubuntu-26.04 is available.
• Prebuilds require enabling in repo Settings → Codespaces (instructions in updated README).