feat: make Oracle mode the default instead of PoC (quality over cost)

[Delqhi]

Motivation

PoC mode (first-pass-wins) is the current default. It cancels all other candidates as soon as one passes the verify-gate. While cost-efficient, this means a fast-but-minimal solution beats a slow-but-superior solution.

For a verification-first system like SIN-Code, quality should be the default, not cost.

Proposal

• Change default mode from PoC to Oracle when fusion.enabled = true
• PoC remains available via fusion.mode = poc (explicit opt-in for cost-sensitive workloads)
• Document the tradeoff: PoC = cheap+correct, Oracle = expensive+best

Config change

fusion.mode = "oracle"  # default: oracle (was: poc)

New config key fusion.mode (string: "poc" | "oracle" | "plan-merge"). Default: "oracle".

Non-breaking

• PoC mode still available via explicit config
• Existing configs without fusion.mode get Oracle (upgrade in quality)
• Documented in AGENTS.md §7 and CHANGELOG

[Delqhi]

Shipped in PR #397 (commit efe6121). Oracle is now the default fusion mode (quality over cost). PoC still available via fusion.mode='poc'. New fusion.mode config: poc|oracle|plan-merge.