fix(opencode): bump pin to 1.17.3 to recover OpenRouter reasoning models

[wangyuyan-agent]

Summary

Bump the Dockerfile.opencode pin from 1.16.2 to 1.17.3 and document
the minimum version required for OpenRouter reasoning models (e.g.
openrouter/google/gemini-3-flash).

Discord Discussion URL: https://discord.com/channels/1491295327620169908/1491365157010542652/1514522869768654868

Root cause

OpenAB consumes opencode's ACP server via stdio JSON-RPC and renders text solely from sessionUpdate: agent_message_chunk notifications (src/acp/protocol.rs::classify_notification). On opencode-ai 1.16.2 — and, based on changelog inspection, the entire 1.16.x line — the ACP server fails to emit agent_message_chunk for OpenRouter reasoning models. The assistant text part is correctly written into opencode's internal session storage (visible via opencode export <session_id>), but never reaches OpenAB. OpenAB's recv loop sees only usage_update followed by a stopReason: end_turn response, runs through compose_display, finds an empty buffer, and falls back to _(no response)_ (src/adapter.rs:660-665).

The upstream fix is sst/opencode#30332 ("fix(opencode): generate reasoning variants for all OpenRouter models"), released in opencode 1.17.0. We pin to the more conservative 1.17.3 because that is the version verified end-to-end by a community reporter (Discord thread linked above).

What this PR does

• Dockerfile.opencode: OPENCODE_VERSION 1.16.2 → 1.17.3, with a comment block explaining the ACP regression so the next person bumping the version knows why we will not regress below 1.17.0.
• docs/opencode.md: append a "Minimum version for OpenRouter reasoning models" bullet to the existing Notes section, linking fix(opencode): generate reasoning variants for all OpenRouter models. anomalyco/opencode#30332.

No Rust source touched. No behavior change inside OpenAB itself — this is a pure dependency floor lift plus documentation.

Verification

• Reporter confirmed end-to-end recovery on openrouter/google/gemini-3-flash after upgrading opencode 1.16.2 → 1.17.3:
  • debug log now shows {"sessionUpdate":"agent_message_chunk","content":{"type":"text","text":"..."}} arriving before the usage_update and the session/prompt response, in the order the OpenAB parser expects.
• npm view opencode-ai@1.17.3 version resolves cleanly on the public registry.
• No Rust source modified, so cargo build/cargo test are unaffected. The Docker builder stage will exercise the new pin via npm install -g opencode-ai@1.17.3 in CI's Dockerfile.opencode runtime stage.
• docs/opencode.md markdown renders correctly in GitHub preview; fix(opencode): generate reasoning variants for all OpenRouter models. anomalyco/opencode#30332 URL resolves to the merged upstream PR.