refactor(host): rename the directory-picker dialog backend and kind to native

The browse interaction also presents a dialog (the in-app modal), so 'dialog'
failed to discriminate the two capability kinds; 'native' names where the
chooser runs. Package directory-picker-dialog -> directory-picker-native, kind
'dialog' -> 'native', with every seam/gateway/client/doc reference updated and
the seam Agent Note's naming rationale rewritten to match.

packages/client/ui-workspace/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-workspace/README.md
-README.md: 58aaf56e1953f00417492d766d8f4ae4a0081c81
-README.zh.md: 7c7b33e0ea68fefee8f857cb5e67a36ec5a854f5
+README.md: deaa25184f5ddbfc5980033ce60cef43577ff33c
+README.zh.md: e14e8ca6a2e3d65ce5fc403291e45ebc03598e04