assets: add Workflows to the social preview, and ship its source (#25)

The card advertised Skills, Hooks, Subagents, MCP, and Agent Teams.
Dynamic Workflows became a headline README section in #20 and was
missing, so the card was already behind the content.

The pills now mirror the README's five extension points exactly:
Skills, Hooks, Subagents, Workflows, MCP. Agent Teams keeps its billing
in the tagline directly above, which is also more honest about it being
experimental while the other five are stable.

Adds Images/social-preview.svg as the editable source. The card was
previously a raster with no source in the repo, so updating it meant
recreating it from scratch. Same reasoning as the skill-resolution
diagram in #24: the asset should be fixable in a PR.

Mascot geometry and the #E07C4C orange are lifted from
Images/claude-jumping.svg so the two stay in sync. Layout, type scale,
and palette otherwise match the previous card.

PNG regenerated at exactly 1280x640, 230KB down to 69KB.

Author: wesammustafa