feat(prompt-harness): centralize all system prompts (#531)

Add a new prompts module to cortex-prompt-harness that provides a single source
of truth for all system prompts used throughout Cortex CLI. This improves
visibility and maintainability of the prompt harness.

## New modules:

- prompts/core.rs: Core agent prompts (main Cortex prompt, TUI prompt)
- prompts/agents.rs: Built-in agent prompts (explore, general, research, etc.)
- prompts/tasks.rs: Task-related prompts (summarization, compaction, titles)
- prompts/review.rs: Code review prompts
- prompts/tools.rs: Tool-specific prompts (subagent executor, mentions)
- prompts/generation.rs: Agent generation prompts

## Key features:

- All prompts are now documented with rustdoc comments
- Helper functions for common operations (build_review_prompt, has_summary_output)
- Type-safe review target enum for different review scenarios
- Comprehensive tests for all prompt modules
- Re-exports for commonly used prompts at module level

Author: echobt

cortex-prompt-harness/src/lib.rs

@@ -6,6 +6,7 @@
 //! - **Dynamic prompt construction**: Build system prompts based on context (tasks, agents, etc.)
 //! - **Agent state tracking**: Track agent state changes between messages
 //! - **Update notifications**: Inject notifications about agent changes into the next prompt
+//! - **Centralized prompts**: Single source of truth for all system prompts (see [`prompts`] module)
 //!
 //! # Architecture
 //!
@@ -28,6 +29,27 @@
 //! └─────────────────────────────────────────────────────────────────────┘
 //! ```
 //!
+//! # Centralized Prompts
+//!
+//! All system prompts are centralized in the [`prompts`] module for better
+//! visibility and maintainability:
+//!
+//! ```rust
+//! use cortex_prompt_harness::prompts;
+//!
+//! // Core prompts
+//! let main_prompt = prompts::core::CORTEX_MAIN_PROMPT;
+//! let tui_prompt = prompts::core::TUI_SYSTEM_PROMPT_TEMPLATE;
+//!
+//! // Agent prompts
+//! let explore = prompts::agents::EXPLORE_AGENT_PROMPT;
+//! let research = prompts::agents::RESEARCH_AGENT_PROMPT;
+//!
+//! // Task prompts
+//! let summarization = prompts::tasks::SUMMARIZATION_PROMPT;
+//! let compaction = prompts::tasks::COMPACTION_PROMPT;
+//! ```
+//!
 //! # Example
 //!
 //! ```rust
@@ -55,6 +77,7 @@
 pub mod builder;
 pub mod context;
 pub mod notifications;
+pub mod prompts;
 pub mod sections;
 pub mod state;
 pub mod tracker;