chore(models): drop image models that shut down in Oct 2026 (Nova Canvas 09-30, gemini-2.5-flash-image 10-02, gpt-image-1 10-23)

[diillson]

Scheduled cleanup after PR #1363, which kept these entries alive with deprecation notes while they still work. After the dates below the providers hard-fail, so the catalog entries (and any now-dead code paths) should be removed:

• 2026-09-30 — amazon.nova-canvas-v1:0 (Bedrock EOL; last Amazon first-party image model — after removal the whole legacy TEXT_IMAGE request path in llm/imagegen/bedrock.go can go too, Stability becomes the only Bedrock image family).
• 2026-10-02 — gemini-2.5-flash-image (Google shutdown; replacement gemini-3.1-flash-lite-image).
• 2026-10-23 — gpt-image-1 (OpenAI shutdown; replacement gpt-image-2, already our default).

Also sweep the docs site (~/chatcli.ai, EN+pt) and regen the corpus.