feat(prompts): implement skill-based dynamic prompt architecture (#21)

feat(prompts): implement skill-based dynamic prompt architecture

This PR transforms the monolithic system prompt architecture into a modular skill-based system where the agent loads specialized skills on-demand based on task requirements. This reduces context window usage by only including relevant instructions.

Changes:
- Add builtin_skills.rs with six built-in skills (git, code-quality, file-operations, debugging, security, planning)
- Add base_agent.rs with minimal base prompt and skill loading mechanism
- Integrate skill system with prompt builder in cortex-engine
- Add built-in skill support to UseSkill handler
- Integrate top_agent module from PR #20

Benefits:
- Reduced context usage (~500 tokens base vs ~2000+ monolithic)
- Modular updates: skills can be updated independently
- Clear organization: instructions organized by domain
- Backward compatible: original monolithic prompt still available

Testing: 200+ tests covering all new functionality

Author: echobt

src/cortex-engine/src/session/handlers.rs

@@ -17,7 +17,10 @@ use crate::rollout::recorder::SessionMeta;
 use crate::summarization::SummarizationStrategy;
 
 use super::Session;
-use super::prompt::build_system_prompt;
+use super::prompt::{
+    USE_SKILL_BASED_PROMPT, auto_detect_skills_from_message, build_system_prompt,
+    build_system_prompt_with_skills, inject_skills,
+};
 
 impl Session {
     /// Handle an incoming submission.
@@ -122,7 +125,12 @@ impl Session {
                 // Update system prompt in existing message history
                 if let Some(msg) = self.messages.first_mut() {
                     if matches!(msg.role, crate::client::MessageRole::System) {
-                        *msg = Message::system(build_system_prompt(&self.config));
+                        let new_prompt = if USE_SKILL_BASED_PROMPT {
+                            build_system_prompt_with_skills(&self.config, &[])
+                        } else {
+                            build_system_prompt(&self.config)
+                        };
+                        *msg = Message::system(new_prompt);
                     }
                 }
             }
@@ -193,6 +201,29 @@ impl Session {
 
         tracing::debug!("User message: {}", user_text);
 
+        // Auto-detect and inject skills based on the user's message (only on first message)
+        // This reduces context window usage by only loading relevant skills.
+        if USE_SKILL_BASED_PROMPT && self.turn_id == 1 {
+            let detected_skills = auto_detect_skills_from_message(&user_text);
+            if !detected_skills.is_empty() {
+                tracing::info!("Auto-detected skills for task: {:?}", detected_skills);
+
+                // Update the system prompt with detected skills
+                if let Some(msg) = self.messages.first_mut() {
+                    if matches!(msg.role, MessageRole::System) {
+                        if let Some(current_content) = msg.content.as_text() {
+                            let updated_prompt = inject_skills(current_content, &detected_skills);
+                            *msg = Message::system(updated_prompt);
+                            tracing::debug!(
+                                "Injected skills into system prompt: {:?}",
+                                detected_skills
+                            );
+                        }
+                    }
+                }
+            }
+        }
+
         // Track messages for potential redo functionality
         let _msg_start_idx = self.messages.len();
 

src/cortex-engine/src/session/lifecycle.rs

@@ -21,7 +21,7 @@ use crate::rollout::{RolloutRecorder, SESSIONS_SUBDIR, get_rollout_path, read_ro
 use crate::tools::ToolRouter;
 
 use super::Session;
-use super::prompt::build_system_prompt;
+use super::prompt::{USE_SKILL_BASED_PROMPT, build_system_prompt, build_system_prompt_with_skills};
 use super::types::{SessionHandle, SessionInfo, TokenCounter};
 
 impl Session {
@@ -64,8 +64,16 @@ impl Session {
         recorder.record_meta(&meta)?;
 
         // Initialize with system prompt
+        // Use skill-based minimal prompt by default to reduce context window usage.
+        // Skills will be loaded on-demand based on task requirements.
         let mut messages = Vec::new();
-        messages.push(Message::system(build_system_prompt(&config)));
+        let initial_prompt = if USE_SKILL_BASED_PROMPT {
+            // Start with the minimal base prompt - skills will be injected on first user message
+            build_system_prompt_with_skills(&config, &[])
+        } else {
+            build_system_prompt(&config)
+        };
+        messages.push(Message::system(initial_prompt));
 
         // Initialize snapshot manager
         let snapshot_dir = config
@@ -145,8 +153,15 @@ impl Session {
         let mut tool_router = ToolRouter::new();
 
         // Rebuild messages from events
+        // For resumed sessions, we still use the base prompt since skills might have been
+        // loaded during the previous session and we want consistency.
         let mut messages = Vec::new();
-        messages.push(Message::system(build_system_prompt(&config)));
+        let initial_prompt = if USE_SKILL_BASED_PROMPT {
+            build_system_prompt_with_skills(&config, &[])
+        } else {
+            build_system_prompt(&config)
+        };
+        messages.push(Message::system(initial_prompt));
 
         let events = get_events(&entries);
         for event_msg in events {
@@ -257,7 +272,12 @@ impl Session {
         let entries = read_rollout(&rollout_path)?;
 
         let mut messages = Vec::new();
-        messages.push(Message::system(build_system_prompt(&config)));
+        let initial_prompt = if USE_SKILL_BASED_PROMPT {
+            build_system_prompt_with_skills(&config, &[])
+        } else {
+            build_system_prompt(&config)
+        };
+        messages.push(Message::system(initial_prompt));
 
         let events = get_events(&entries);
 

src/cortex-engine/src/session/mod.rs

@@ -22,7 +22,10 @@ use crate::rollout::RolloutRecorder;
 use crate::tools::ToolRouter;
 
 pub use lifecycle::list_sessions;
-pub use prompt::build_system_prompt;
+pub use prompt::{
+    USE_SKILL_BASED_PROMPT, auto_detect_skills_from_message, available_skills, build_system_prompt,
+    build_system_prompt_with_skills, inject_skills, is_valid_skill,
+};
 pub use types::{SessionHandle, SessionInfo, TokenCounter};
 
 /// A running session that handles conversation with the model.