[HIGH] Plan Mode is file flag only, no state machine enforcement

[tuterx]

EnterPlanMode/ExitPlanMode just flip a file flag. No read-only enforcement, no structured plan display, no approval requirement before executing.

Fix: Implement Plan Mode state machine: block writes, display plan, require user approval to proceed.

[1716775457damn]

Plan Mode as just a file flag is too weak. Key recommendations: (1) implement write-blocking layer that intercepts file ops during planning, (2) render the plan in a structured format for review, (3) add an explicit approval gate before executing. State machine: IDLE -> PLANNING -> PLAN_READY -> EXECUTING -> COMPLETE would be a clean approach.

[Yeachan-Heo]

Tracked in ROADMAP.md as an architectural gap. This is a known limitation, not a bug.