feat(pricing): add OpenCode Go model entries to catalog

[bravo1goingdark]

Summary

Adds 14 OpenCode Go model slugs to the bundled LiteLLM-derived pricing catalog. OpenCode's curated set isn't in upstream LiteLLM yet (their slugs lag releases by a few weeks), so we use sibling-family rates as documented approximations.

OpenCode slug	Rate source	$/M in / $/M out
`opencode/glm-5.1`, `opencode/glm-5`	`baseten/zai-org/GLM-5`	0.95 / 3.15
`opencode/kimi-k2.6`, `kimi-k2.5`	`bedrock/moonshotai.kimi-k2.5`	0.60 / 3.03
`opencode/minimax-m2.7`, `m2.5`	`bedrock/minimax.minimax-m2.5`	0.36 / 1.44
`opencode/deepseek-v4-{pro,flash}`	`azure_ai/deepseek-v3.2`	0.58 / 1.68
`opencode/qwen3.{5,6}-plus`	`bedrock/qwen.qwen3-coder-next`	0.60 / 1.50
`opencode/mimo-v2-{pro,omni,2.5,2.5-pro}`	`openrouter/xiaomi/mimo-v2-flash`	0.09 / 0.29

Keys are prefixed `opencode/` to group with the rest of the provider's catalog, mirroring how mistral/groq entries are stored. The catalog's existing prefix-strip during lookup resolves the bare slug.

Cost reported is upstream-equivalent value, not what the user actually pays — OpenCode Go is a flat-rate $10/mo plan. Useful for tracking $60/mo cap consumption and comparing against direct-provider pricing. Obol's docs will call this out as approximate.

Test plan

• `cargo test -p keplor-pricing` — 12 passed
• `cargo build --release --bin keplor` clean
• After deploy: fire glm-5.1 request via Obol, query `/v1/events`, confirm `cost_nanodollars` is non-zero