Add devcontainer for faster GitHub Codespaces setup

[sejas]

Related issues

How AI was used in this PR

• AI helped to write the dev container definition

Proposed Changes

I suggest adding a GitHub dev container definition to launch GitHub Codespaces easily. It just installs the correct node version and project dependencies. We can consider installing more things like Claude CLI, or creating alias.
I find it especially useful for code reviews.

Testing Instructions

• Create a codespace from this branch (Code → Codespaces → + Create a codespace on add-devcontainer)
• Wait for postCreateCommand to finish
• Run node apps/cli/dist/cli/main.mjs --help — should print CLI help without any manual setup
• node --version should report v24.15.0

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[epeicher]

Thanks for improving this @sejas! This will help with reviews and quick testing. I have tested it, and it works as expected, I can see the CLI command output and the expected node version. LGTM!

[epeicher]

Nit: I don't see the output of node apps/cli/dist/cli/main.mjs --help so maybe that command can be avoided? I don't mind having that it there either

[wpmobilebot]

📊 Performance Test Results

Comparing be5308c vs trunk

app-size

Metric	trunk	be5308c	Diff	Change
App Size (Mac)	1411.92 MB	1411.89 MB	0.03 MB	⚪ 0.0%

site-editor

Metric	trunk	be5308c	Diff	Change
load	1122 ms	743 ms	379 ms	🟢 -33.8%

site-startup

Metric	trunk	be5308c	Diff	Change
siteCreation	6515 ms	6502 ms	13 ms	⚪ 0.0%
siteStartup	2383 ms	2398 ms	+15 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)