marketing: Workspaces overview rewrite, the coordination-layer frame (#1607)

* marketing: rewrite the Workspaces overview page

Lead with shared documents that people and coding agents plan, review and
decide in, with Git-backed history, and agents as first-class users through
the API, CLI and MCP. Five short sections replace the context-sharing framing.
Fix stale copy: the review step no longer says 'Plans and code', the success
note names the Organization plan instead of 'Teams', the meta description
drops 'design partner', and the open source note no longer calls Workspaces a
layer around the core. The waitlist form, its fields, script and endpoint are
unchanged; dead phase and intro styles are removed.

* marketing: reframe the Workspaces page as the shared coordination layer

Owner direction: the shared coordination layer in the agentic software
factory era, for planning, review, shaping work and keeping humans in the
loop. New headline and lead; the five sections re-keyed to planning, review,
shaping the work, people in the loop, and Git plus agent access as the
foundation; the strip reads Plan, Shape, Build. The waitlist form and
scripts are unchanged.

* marketing: leaner Workspaces headline, drop the diagram

Owner look: the headline becomes 'The coordination layer for agent planning
and review.' at a smaller page-title size, and the lead carries the rest. The
Plan, Shape, Build diagram and its styles are removed so the hero and the
form balance. Sections, earlier fixes and the waitlist form are unchanged.

* marketing: trim the Workspaces meta description to 147 characters