feat: Fusion plan-merge mode (N planners → unified plan → 1 coder)

[Delqhi]

Motivation

SIN Fusion v1 has two tournament modes: PoC (first-pass-wins) and Oracle (judge picks best). Both discard N-1 outputs entirely. For planning, discarding is wasteful — the best plan is often a combination of insights from multiple models.

Proposal: ModePlanMerge

1. N models produce plans in parallel (same prompt, different models)
2. LLM judge merges best insights into a Unified Plan
3. One model (worker or judge-selected) codes the Unified Plan
4. Verify-gate validates the result (M3 sacred)

Why plan-merge works where code-merge doesn't

• Plans are natural language — easy to merge semantically
• Code has incompatible abstractions/naming/interfaces — merging creates bugs
• A merged plan captures edge cases that any single model missed
• The verify-gate catches bad plans downstream anyway

Config

Key	Default	Purpose
fusion.plan_merge	false	Enable plan-merge mode
fusion.plan_merge_judge	SIN_EVALUATOR_MODEL	Judge that merges plans

Tournament mode selection

if fusion.plan_merge → ModePlanMerge (plan → merge → code → verify)
else if fusion.oracle_mode → ModeOracle (all code → judge → best)
else → ModePoC (first correct pass wins)

Non-breaking

• Additive: new ModePlanMerge constant, new code path in tournament.go
• Existing PoC and Oracle modes unchanged
• Default off (fusion.plan_merge = false)

Files to create/modify

• cmd/sin-code/internal/fusion/plan_merge.go — new
• cmd/sin-code/internal/fusion/plan_merge_test.go — new
• cmd/sin-code/internal/fusion/tournament.go — add ModePlanMerge dispatch
• cmd/sin-code/internal/loopbuilder/builder.go — wire plan_merge config
• cmd/sin-code/internal/config.go — add FusionPlanMerge field
• cmd/sin-code/fusion_cmd.go — show plan_merge in status/config output

[Delqhi]

Shipped in PR #397 (commit efe6121). ModePlanMerge: N planners → judge merges → 1 coder → verify-gate. 10 tests race-clean, byte-stable.