prompt audit: trim system prompt, kill dead code, improve maintainability
- System prompt: ~200 → ~80 lines (removed tool-first table, API docs,
output protocol, dead 9P entries)
- Removed AllowTools entirely (field, RPC, configs)
- Removed PRIME_* env vars, replaced with typed struct fields
- Merged tool listing + docs into single renderTools()
- Killed BuildToolListing, changed OnToolsChanged to func() signal
- Typed Preamble.Section (compile-time safety)
- Added protocol docs on extractToolResult
- Added session/ package godoc
- Moved output protocol to per-agent prompts
- Rewrote data/agents/README.md
- Moved 7 reference docs from doc/resources/ to doc/
- Deleted PromptEnv() dead function
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