Feat/cs/polish 9

[ReagentX]

• Consolidated protocol and session JSON construction and optional-field handling.
• Moved wire framing from terminal::frame to the top-level frame module.
• Centralized harness command detection and resume generation around each harness’s command shape.
• Embedded rename and group-picker target IDs in App::Mode, eliminating parallel state fields.
• Unified centered-overlay rendering for peek and controls.
• Consolidated task retirement, PID parsing, liveness checks, clipboard selectors, input encoding, and UTF-8 byte truncation.
• Made recovery-write deduplication track the actual written snapshot path.
• Added automatic scratch-directory cleanup for tests.
• Split README fixture generation out of the main app test file.