Add figma-starter extension submitted by @vibhus to:
- extensions/catalog.community.json (alphabetical order)
- docs/community/extensions.md community extensions table

Closes #3545

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…talog entry

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…fallback

The PowerShell update-agent-context.ps1 script falls back to Python when
PS 5.1 (which lacks ConvertFrom-Yaml and cannot parse YAML as JSON) reads
the extension config.  It previously launched Python twice: once to verify
that Python 3 + PyYAML were available, and once to run a temp script file
that parsed the YAML and printed JSON.

On Windows CI each Python startup—plus potential Windows Defender scanning
of a freshly-created .tmp file—can take several seconds.  With two launches
plus PS 5.1's own startup time the subprocess.run(timeout=30) threshold in
test_powershell_script_discovers_nested_plan was regularly exceeded, causing
the CI job to fail.

Replace the two-phase approach with a single Python -c one-liner that
verifies PyYAML availability, parses the YAML file, and emits JSON in one
process.  This halves the number of Python launches and eliminates the temp
file entirely, keeping total execution time well under 30 s.

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…zation state

The runtime optimization (one-liner Python fallback, stderr suppression) was
unrelated to the Figma Starter catalog addition (issue #3545) and removed the
actionable PyYAML/parse diagnostic messages. Revert the file so the PR only
contains the catalog-entry and documentation changes.

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