Preserve dirty Copy and Chat overlays during Agent Models changes

[baron]

Summary

• stop direct Agent Models changes from broadcasting the broad workspace-scoped recommendationsDidApply notification
• rely on the scoped agentModelsSettingsDidChange propagation path
• add a regression test proving dirty Copy and Chat window overlays remain intact

Review

• planned with a dedicated worktree implementer
• consulted Fable after planning
• consulted Fable again after implementation; review feedback was applied
• publication commit was recreated exactly on current origin/main while preserving the reviewed source/test blobs

Validation

• GlobalSettingsCrossWindowPropagationTests: 4 tests, 0 failures on current main
• make dev-lint: passed; SwiftFormat check reported 0/1456 changes and strict SwiftLint passed
• mandatory commit and push preflights: passed, including guardrails and outgoing-range secret scan
• git diff --check: clean

Validation boundary

A monolithic local pr-ready root lane on the earlier base hit the 3600-second limit in unrelated codemap suites. The publication branch was refreshed to current main, where the focused regression and lint are green. Please require exact-head hosted merge-ref CI before merge.

Live/debug app validation is intentionally deferred for this source-only fix.