Create docs/plugin-ecosystem(.en).md as the ecosystem manifesto: an open,
composable, sustainable vision with composition-first, declare-clearly,
and compatibility-first principles, the desktop shell as the first
example, living-document community governance, and the marketplace as
the trust signal.
The root README intro now ends with the co-progress vision and links the
manifesto; the docs index and root README tables list it; the
plugin-development ecosystem-vision section points to it as the full
statement.
