Real-coder layer: OpenCode via ACP — full rebuild + auto-wire

[seamus-brady]

Summary

• Replaces the unfinished REST-based coder integration with an ACP-driven (Agent Client Protocol over JSON-RPC stdio) layer talking to a sandboxed OpenCode 1.14.25 container. ~1500 LOC of REST plumbing out, ~400 LOC of ACP in.
• dispatch_coder is the cog/PM tool: one OpenCode session per call, async via OTP worker so the cog stays responsive — mid-flight cancel_coder_session is reachable. agent_coder collapses to a thin specialist (frame → dispatch → inspect → land), only registered when [coder] is fully configured.
• Auto-wire — fresh installs just work: [coder] defaults are populated (image=springdrift-coder:1.14.25, provider=anthropic, model=claude-sonnet-4-6); project_root falls back to cwd; the application auto-builds Containerfile.coder on first boot if the image is missing. Operator only needs ANTHROPIC_API_KEY in .env.

What's in the box

• coder/acp.gleam — JSON-RPC over stdio bindings; pure decoders pinned against probed shapes
• coder/manager.gleam — OTP actor with warm container pool, per-dispatch driver, three-stage kill chain, per-task budget enforcement, hourly cost cap, janitor TTL
• coder/ingest.gleam — completed dispatch → CbrCase (CodePattern, tools_used = ACP tool titles seen during the session) + raw JSON archive in .springdrift/memory/coder/sessions/
• tools/coder_dispatch.gleam — async-via-worker dispatch_coder + sync cancel_coder_session / list_coder_sessions; budget clamp-and-report against operator ceilings
• agent/cognitive*.gleam — PendingCoderDispatch + CoderDispatchComplete ride the existing WaitingForAgents accumulator; multiple parallel dispatches fold into one rethink

Removed

• coder/client.gleam REST wrapper, coder/supervisor.gleam per-task slot lifecycle, http_probe_get FFI
• run_tests / run_build / run_format host-side scaffolding tools
• coder_start_session / coder_send / coder_end_session tools, replaced by dispatch_coder
• ProjectCommands type + default_project_commands
• Three-mode coder agent branching: sandbox-only and no-sandbox legacy modes are gone
• manager.acquire / manager.send / manager.end_session deprecated compat shims

Test plan

• gleam build clean
• gleam format clean
• gleam test — 2126 passing (gained 49 from rebase on top of Fix three agent-tool dispatch bugs of the same family #163–Sub-agent truncation guard with embedded partial output #166)
• Rebased on origin/main with conflict in cognitive/agents.gleam resolved — kept both Fix three agent-tool dispatch bugs of the same family #163's builtin_calls partition and the new coder_sync_calls partition
• gleam run smoke — confirm boot on the live agent
• scripts/e2e-coder.sh — real OpenCode container round-trip, costs a couple of cents, gated by SPRINGDRIFT_CODER_E2E=1
• Fresh-install smoke: ANTHROPIC_API_KEY only in .env, gleam run, confirm image auto-builds and dispatch_coder is on the cog tool surface

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