feat: add the DeepSeek platform provider and deepseek-flash

Refs #83

The first vendor-cloud chat row in agent/models.yml. The provider label is the
bare word `deepseek` because pi-catalog/src/hosts.ts keys the direct-DeepSeek
behaviour set (max_tokens field, thinking extraBody, thinking-disable mode) off
that exact name and off api.deepseek.com; the house `<api> (<host>)` pattern
would not match and the row would lose the quirks the endpoint answers to.

The key is command-backed (`!cat ~/.config/secrets/deepseek_api_key`, 600), so
no sk-… value enters the tracked tree, and the row declares the model card's own
pair: contextWindow 1048576 / maxTokens 262144, input [text, image], with the
October 2026 peak rate card in cost. Docs move to the new
omp-docs/core/deepseek-provider.md, indexed and logged in the same commit.

omp-docs/core/index.md

@@ -16,6 +16,7 @@ These files define the structural conventions that govern the agent system itsel
 * [OMP Status Line Customization](statusline.md) — How presets, composer shapes, and segment arrays actually interact; the fleet's tuned bar. (moved from /pi/, 2026-10-19)
 * [OMP Subagent Limits & Fan-out Policy](subagent-limits.md) — Live caps: concurrency 2, no nested spawns (the `maxRecursionDepth: 0` trap), per-endpoint in-flight ceilings, isolation policy. (moved from /pi/, 2026-10-19)
 * [WAN Model Access](wan-model-access.md) — Driving the fleet's models from outside the home network through the OpenWebUI door: the command, the key file and its rotation law, what the door publishes, and the off-network failure modes.
+* [DeepSeek Platform Provider](deepseek-provider.md) — The first vendor-cloud row in `agent/models.yml`: why its label is the bare word `deepseek`, the key file, and the window/maxTokens/cost values it declares.
 * [OMP Snapcompact — Bitmap-Frame Compaction](snapcompact.md) — What shape frank-archon actually reads, the measured recall/billing table, the `imageCountBudget` knob and the pricing that isn't, and the 85 % trigger law.
 
 **Change log:** the bundle keeps exactly one, one level up — [Bundle Change Log](../log.md). A directory here never has its own.