upstream: per-role instructionsFile injected into session system prompt

[polymath-orchestrator]

Goal

Upstream-shaped feature (vanilla rule: issue here + upstream PR, we carry the delta): add a per-role instructions file to ao's project config so the daemon injects project-specific standing instructions into a session's system prompt by role — orchestrator vs worker — instead of projects smuggling role policy through the shared repo CLAUDE.md.

Proposed shape:

// ProjectConfig
"orchestrator": { "agent": "...", "instructionsFile": ".claude/orchestrator-policy.md" },
"worker":       { "agent": "...", "instructionsFile": ".claude/worker-policy.md" }

RoleOverride (backend/internal/domain/projectconfig.go) currently carries only harness + agent config. Add InstructionsFile (path relative to the project root, absolute allowed); buildSystemPrompt (backend/internal/session_manager/manager.go) reads and appends its contents after the built-in per-kind prompt for that SessionKind, at spawn and restore (restore already recomputes prompts). Missing/unreadable file: log a warning, spawn without it — never block a spawn.

How we got here

ao already types sessions (KindWorker/KindOrchestrator) and injects hardcoded per-kind system prompts. But there is no vanilla slot for a project's own standing policy per role, so we put orchestrator policy into the repo CLAUDE.md — which every session in the checkout loads — and an interactive session acted on it and double-dispatched workers (2026-07-06 incident). The interim fix is a prose pointer in CLAUDE.md keyed on the injected role prompt; this feature makes the injection native and deletes the pointer. Upstream is already moving this direction (AgentWrapper#2361 enriched the coordinator prompt; AgentWrapper#2444 custom AO agent prompts for Kiro).

Acceptance criteria

• TDD: RoleOverride.InstructionsFile in domain config with validation (path sanity) + tests
• buildSystemPrompt appends file contents per kind at spawn and restore; missing file degrades gracefully (warn, continue); tests cover both kinds + restore path
• OpenAPI spec / DTOs / frontend schema regenerated where project config is exposed
• Backend gates green: cd backend && go build ./... && go vet ./... && go test ./...
• Verified live: set an instructions file for the orchestrator role on a test project, spawn, confirm the text reaches the session's system prompt
• Upstream PR opened against AgentWrapper/agent-orchestrator; our PR references it (pattern: Implement per-session --model on ao spawn (upstream-shaped) #1 per-session --model, Add codex-fugu agent adapter (upstream-shaped, like #1) #3 codex-fugu)
• PARK the merge for a human: touches backend/internal/session_manager/** (sensitive path — autonomous merge forbidden); report merge-ready instead

Context / pointers

• backend/internal/domain/projectconfig.go (RoleOverride), backend/internal/session_manager/manager.go (buildSystemPrompt, orchestratorPrompt, workerOrchestratorPrompt)
• Depends conceptually on the trackerIntake+policy-restructure ticket (provides .claude/orchestrator-policy.md); no hard ordering
• Priority: P2 · type: feature