fix(setup): keep post-edit explicitly opt-in

[tt-a1i]

Summary

• keep the existing empty post-edit command as the canonical default-off state
• tell the episode-scoped setup tool to set a non-empty command only when the current user explicitly asks for Post-edit
• preserve unrelated and existing setup values when the field is omitted
• independently bound the displayed command and stderr/stdout in failure notifications
• bound rejected execution errors before they reach the TUI
• cover first-run off, explicit enable/disable, and unrelated-update preservation through the real typed setup tool

The runtime does not add a keyword parser: natural-language intent remains model-owned, while the model-visible field contract carries the opt-in rule.

Validation

• focused setup/post-edit suite: 25 passed
• bun run check
• bun run test: 900 Node tests and 30 Vitest tests passed

Closes #136