feat: share env files in --demo

Providers whose containers are files give their contents through
provider.Contents, which sharing reads instead of the disk. The demo
repos build their env files from their keys, and the second demo
account has a key to share with (kv-shared-prod / env-share), so S → f
and opening the link work in --demo; the demo repos aren't on disk,
so opening asks where to save.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/development.md

@@ -7,6 +7,11 @@ go test ./...
 go test ./internal/ui/app -run TestGolden -update   # after intentional UI changes
 ```
 
+In `--demo`, sharing env files works end to end: `S` → `f` on a demo
+repo, pick `kv-shared-prod / env-share` (the second account's) as the
+key, then paste the link back. The demo repos aren't on disk, so opening
+asks where to save, as for a repo you haven't cloned.
+
 A stripped binary is about 21 MB instead of 30 MB:
 
 ```sh