feat: ✨ LiteLLM multi-provider model selection + catalog-driven UI

[nj-io]

What

Replaces the hand-rolled OpenAICompatClient with a LiteLLM-backed LiteLLMClient (openai/openrouter/ollama) behind the unchanged LLMClient.complete() interface, and makes the model catalog the single source of truth wired to the settings UI. Goal: select OpenRouter models like openrouter/z-ai/glm-5.2 reliably, with real per-call cost tracking.

Adopts the proven pattern from mirra's PromptLab (LiteLLM + registry-as-cost-truth), adapted to this project's tool-calling contract.

Key changes

• llm/litellm_client.py (new): forces the single expected tool; on a tool-less reply, parses JSON from the message body, else returns a text block (never raises) so the Gatekeeper's prose fallback is preserved; real cost via LiteLLM response_cost → catalog fallback → honest unknown, with a cost_source field.
• catalog.py: single source of cost truth (estimate_cost_cents, get_all_models, get_model_by_full_id); refreshed to current models (Opus 4.8 / Sonnet 5 / Haiku 4.5 / Fable 5) + verified OpenRouter models incl. z-ai/glm-5.2.
• client.py (Anthropic, same transport) now sources cost from the catalog — fixes silent $0 cost for current model IDs. claude-cli reports cost_source="subscription" ($0), isolated for easy change if subscription calls ever bill.
• parsing.extract_json_object: shared boundary JSON parser (promoted from a private claude_cli helper that dragged a Unix-only pwd import into the OpenRouter path).
• Web: GET /api/models endpoint + OpenRouter key validation; the settings Models section is now catalog-driven (provider-grouped dropdown showing cost/context) instead of three drifting hardcoded lists. Free-text provider/model-id retained.
• ClaudeClient / ClaudeCliClient untouched (Anthropic SDK + claude -p subscription path stay as-is); litellm is an optional dep (multi-provider extra, pinned >=1.91.0).

Verification

• ruff + mypy clean; test suite green (new test_litellm_client.py; updated catalog/factory/llm_client/claude_cli; test_openai_compat_client.py removed).
• Live openrouter/z-ai/glm-5.2 call through the factory: real tool call + cost 0.0505¢, source=provider.
• Browser UI round-trip: catalog dropdown → select → auto-save → persisted to config.yaml.

Notes

• Plan-reviewed before implementation (an adversarial review caught the Anthropic $0-cost bug, the pwd import leak, and a Gatekeeper regression — all fixed here).
• Branched from clean develop HEAD, so it is isolated from unrelated in-progress media/buttons work in the local tree.
• Doc updates (REUSABILITY.md, docs/CLAUDE.md) live in the separate docs repo and are committed there, not in this PR.

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