fix: stable provider selection — re-roll only on fresh session creation

[alexsiri7]

Upstream design discussion: #68 (comment: #68 (comment))

Summary

Provider rotation is merged but unusable. RandomStrategy.Select() in resolveTemplate runs every reconcile tick → different pick → config-drift restart loop.

Fix

1. Store selected provider in bead.Metadata["provider"] at creation
2. On reconcile: read stored provider from existing bead, skip re-roll
3. Quota-kill via gc session close forces fresh bead → new random pick
4. Add existingProvider param to resolveTemplate to bypass random selection for running sessions

Key Files

• cmd/gc/template_resolve.go:92 — resolveTemplate calls ResolveProvider every tick
• cmd/gc/build_desired_state.go:403 — discoverSessionBeads passes to resolveTemplate
• cmd/gc/session_template_start.go — materialize stores provider in bead
• internal/config/provider_strategy.go — RandomStrategy