Registry: env constraints (env:emdash, env:astro) end-to-end with admin compatibility warning #1031

Plugins shouldn't install on hosts they don't support. The lexicon
already defines release.requires for env:emdash, env:astro,
and (forward-compat) future env keys.

Parent: #1026

CLI

• release.requires in the manifest, keyed by env:*, value is a
  semver range string. Validated at parse time against the semver
  range grammar.
• Embedded in the release record at publish.

Admin

• Resolve runtime env values from the running EmDash install
  (already available via emdash.version and the Astro version from
  the integration).
• RegistryPluginDetail.tsx: if any requires entry doesn't
  satisfy the running env, show a warning notice ("This plugin
  requires Astro ≥4.16; you're on 4.12. Upgrade Astro before
  installing.") and disable the Install button.
• Server-side: install handler refuses with ENV_INCOMPATIBLE if
  any requires entry doesn't satisfy. Belt-and-braces vs admin
  bypassing the UI gate.

Acceptance

• CLI publishes a release with env:emdash and env:astro constraints.
• Admin disables install when host doesn't satisfy the constraints.
• Server refuses install with ENV_INCOMPATIBLE when bypassed.
• Compatible constraints install cleanly.