Add execution suggestion after validation phase

[mmorris35]

After DevPlan completes the validation phase, the output should explicitly suggest using Haiku as the executor.

Suggested output:

Plan ready for execution. Exit this session and start a fresh one with Haiku loaded, then ask Claude to implement the phases.

Why:

• Formalizes the optimal workflow discovered through trial-and-error
• Makes the Haiku executor pattern discoverable
• Users shouldn't have to learn the hard way that Opus overthinkes simple tasks

Context:

• Using Haiku directly for execution is significantly faster than Opus orchestrating Haiku
• Opus tends to second-guess and overthink even simple implementation tasks
• The session restart overhead (~10-20s) is trivial compared to the time/cost savings

This should appear after the validation phase completes, guiding users toward the most efficient execution path.