[H-4 / Phase 2] [MEDIUM] Upgrade Anthropic default model to Sonnet 4.6

[matthewod11-stack]

Full spec: docs/hardening-roadmap-2026-04-16.md#h-4

Description

packages/ai/src/provider-anthropic.ts:15 pins claude-sonnet-4-20250514. Current family is 4.x (Opus 4.7, Sonnet 4.6). sourcerer init lands users on stale capabilities.

Current State

• DEFAULT_MODEL = 'claude-sonnet-4-20250514' in provider-anthropic.ts:15.
• sourcerer config status doesn't show effective model.

Suggested Fix

• DEFAULT_MODEL = 'claude-sonnet-4-6' (Sonnet 4.6 is the right default for per-candidate scoring).
• Add effective-model display in sourcerer config status.
• Document model selection in README (Opus 4.7 deep narrative + 1M batch, Sonnet 4.6 default, Haiku 4.5 bulk).

Verification

• pnpm build passes
• pnpm test passes
• pnpm typecheck clean
• sourcerer config status prints effective model name
• README has "Model selection" subsection

Automation Hints

scope: packages/ai/src/provider-anthropic.ts, apps/cli/src/commands/config-status.ts, README.md
do-not-touch: other adapters
approach: config-change
risk: low
max-files-changed: 3
blocked-by: none
bail-if: AI provider tests fail

Priority

Medium