fix(opencode): use instructions array, not agent.build.prompt

[chubes4]

Summary

Switch runtimes/opencode.sh away from emitting agent.build.prompt / agent.plan.prompt with a {file:}-bundle, and over to a top-level instructions: [...] array.

Root cause

Setting agent.build.prompt in opencode.json puts our custom prompt at the very top of system[1] in the outgoing Anthropic request, which pushes OpenCode's canonical <environment>…Skills provide specialized instructions… opening down.

Anthropic's third-party-app detector fingerprints the first bytes of system[1] on Claude Max OAuth requests. When the canonical opening isn't the first thing, the request is routed through extra-usage billing and returns HTTP 400:

{"error":{"type":"invalid_request_error",
 "message":"Third-party apps now draw from your extra usage,
            not your plan limits. Add more at
            claude.ai/settings/usage and keep going."}}

This bricks every Opus-tier Claude Max request the moment any memory file is attached via agent.build.prompt, even a tiny one (repro: 251-byte USER.md is enough).

Tied to remorses/kimaki#99 — Kimaki 0.4.101 fixed the system-prompt sanitization for its request path, but wp-coding-agents' hand-written agent.build.prompt bypasses the sanitizer's anchor and re-breaks it.

Fix

runtime_generate_config() now emits "instructions": ["./path1", "./path2", ...] from $DM_AGENT_FILES. OpenCode loads each path through Instruction.system() in packages/opencode/src/session/instruction.ts and appends them with the Instructions from: <path> prefix after the canonical opening — the same mechanism that auto-discovers AGENTS.md. OAuth flow stays intact; memory is preserved end-to-end.

Also drops the now-unused OPENCODE_PROMPT builder in runtime_discover_dm_paths (AGENTS.md auto-discovery covers the root file; memory files ride the instructions array).

Test plan

• Reproduce 400 on Opus-4-7 with agent.build.prompt = "{file:./AGENTS.md}\n{file:...}" on live intelligence-chubes4 (Kimaki 0.6.0, opencode 1.14.20)
• Apply the equivalent manual fix to opencode.json on the live site → 200 OK, full memory in context (AGENTS.md + SITE.md + RULES.md + SOUL.md + USER.md + MEMORY.md all present in system[])
• bash -n runtimes/opencode.sh
• Fresh setup on a clean site: setup.sh produces opencode.json with instructions array, no agent block
• Kimaki + Opus-4-7 flow works end-to-end on the regenerated config

🤖 Generated with Claude Code