Bump Sonnet 5 to standard pricing (3×, $3/$15) — merge just before 2026-09-01

[khalilgharbaoui]

claude-sonnet-5 shipped in v0.9.3 with its real introductory pricing: $2/M input, $10/M output, shown as (2×) in the picker. That pricing expires 2026-08-31; standard pricing ($3/M in, $15/M out, 3×) begins September 1.

Plan (also recorded in AGENTS.md):

• Open a PR switching claude-sonnet-5 from sonnet5Cost to sonnetCost and multiplier: 2 → 3 in src/models.ts
• Update the README model table row and the pricing paragraph
• Update test-config-models.ts assertions (Claude Sonnet 5 (3×), cost fields)
• Leave the PR open, merge just before Sept 1, release as a patch

Intro pricing was added by @broskees in #18.