Refresh Gemini model and post-0.6.5 docs

[lukacf]

Summary

• replace the featured/recommended Gemini text model with gemini-3.5-flash in the catalog, default provider model, tests, and examples
• refresh public docs and .claude/skills/meerkat-* guidance from the stale 0.6.5/0.6.6 snapshot to the current 0.6.19 release line
• update contract/version snippets and post-0.6.5 architecture notes covering WorkGraph, Azure OpenAI, HTML output, typed notices, mob/runtime durability, and release hardening

Verification

• ./scripts/repo-cargo test -p meerkat-core model_profile
• ./scripts/repo-cargo test -p meerkat-gemini test_build_request_body_with_thinking_level
• ./scripts/repo-cargo test -p meerkat-gemini test_openai_reasoning_effort_provider_tag_does_not_affect_gemini_body
• make docs-check
• make check
• push hooks: cargo fmt check, changed-crate clippy, machine codegen/verify, generated-header audit, bridge gate, Bazel freshness, deterministic gate

Gemini source checked against Google AI docs for gemini-3.5-flash (stable model code, last updated 2026-05-19).

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve backward compatibility for Gemini 3 Flash Preview ID

Do not replace the existing Gemini capability ID in place; this makes gemini-3-flash-preview uncatalogued and breaks exact-match flows that depend on catalog membership. Provider/model resolution and capability lookup are strict (profile_for/capabilities_for only succeed for catalog rows), so existing configs and tests still using gemini-3-flash-preview (for example meerkat-core/src/config.rs and meerkat/src/service_factory.rs) will now fail provider inference or capability-based behavior. Keep the previous ID as a supported row (or alias) and add gemini-3.5-flash separately.

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