fix: deliver generated system prompt files

[whoisasx]

Refs #2272

Summary

• Materialize generated session system prompts under the AO data dir at prompts/<session-id>/system.md.
• Pass both SystemPrompt and SystemPromptFile through fresh launch, restore, and restore fallback launch configs so file-based adapters receive AO standing instructions.
• Add the issue-only fallback task prompt for worker spawns with --issue and no explicit prompt.
• Teach Claude Code restore to prefer the generated prompt file before falling back to inline prompt text.

Tests

• git diff --cached --check
• go test ./internal/session_manager
• go test ./internal/adapters/agent/claudecode
• go test ./internal/adapters/agent/... ./internal/session_manager

Known risks / follow-ups

• This is a focused prompt-delivery slice of Enrich worker and orchestrator system prompts #2272; tracker-backed issue detail injection, project-level agentRules / agentRulesFile / orchestratorRules, and richer project-context prompt templates remain follow-up work.
• Adapters with no native system-prompt support still cannot consume the generated prompt file until adapter-specific support is added.
• Full go test ./... was not rerun on this final branch; focused shared-port and adapter coverage passed.