Fix Rentry CodeMirror input routing

[bartekplus]

Summary

• route CodeMirror-backed editor input through the hidden backing textarea while keeping the visible contenteditable surface for focus and edits
• remove the CodeMirror-specific replacement branch and keep scoped contenteditable edits on the generic DOM path
• keep the new observability coverage and add regression tests for backing-input discovery and listener wiring

Validation

• bun run lint
• bun run format:check
• bun test tests/TextTargetAdapter.test.ts tests/SuggestionLifecycleController.test.ts tests/ContentEditableAdapter.test.ts tests/SuggestionEntrySession.test.ts tests/content_script.behavior.test.ts tests/SuggestionManagerRuntime.test.ts tests/SuggestionManager.test.ts
• bun tsc --noEmit

[bartekplus]

@claude review!