feat: inherit embedded AGENTS.md operating rules for sub-agents

[petabridge-netclaw]

Problem

Sub-agents spawned via spawn_agent do not receive the embedded AGENTS.md operating rules layer that main-agent sessions get. They only receive their own markdown body, optional project instructions, and the hardcoded HeadlessExecutionContract.

This means sub-agents operate without the core safety guardrails that main agents inherit:

• Grounding rules (e.g., "Never state runtime facts without checking with a tool")
• Search/citation policy (inline hyperlink requirements)
• Safety boundaries (Resource Hard Deny awareness)
• Proactive check-back expectations
• Search decision rules

Solution

Sub-agents now inherit the embedded AGENTS.md operating rules alongside their existing context:

Changes

ISystemPromptProvider.cs

• Added GetOperatingRules(TrustAudience) to the interface
• Implemented in FileSystemPromptProvider (substituted embedded AGENTS.md for Team/Personal, null for Public)
• Added stub null implementations to StaticSystemPromptProvider and NullSystemPromptProvider

SubAgentProtocol.cs

• Added OperatingRules property to SubAgentDefinition record

SubAgentSpawner.cs

• Added ResolveOperatingRules() that calls _promptProvider.GetOperatingRules(context.Audience)
• Passes operating rules into SubAgentDefinition at spawn time

SubAgentActor.cs

• Updated BuildSystemPrompt() to assemble the full identity stack:
  1. Embedded AGENTS.md via SystemPromptAssembler.Assemble(agents: definition.OperatingRules, ...)
  2. Sub-agent markdown body + skill overlay appended on top
  3. HeadlessExecutionContract always appended at the bottom

Notes

• SOUL.md and TOOLING.md are intentionally excluded — sub-agents don't need personal identity or redundant tooling info (tool index already available)
• This is a prompt-layering change only — no API or protocol changes
• No breaking changes — operating rules fall back to null gracefully
• closes Sub-agents missing embedded AGENTS.md (operating rules, grounding, safety policy) #1489