Enhance tool documentation and add new features

- Added module-level documentation for memory tools (`remember`, `recall`, `forget`) to clarify their purpose.
- Improved documentation in `recall.rs` and `remember.rs` to describe the functionality and flow of memory entry operations.
- Updated `mod.rs` to include descriptions for the tool trait and execution context.
- Enhanced `plan.rs` with detailed comments on plan-mode signaling tools.
- Documented text search tools in `search.rs` to explain their functionality.
- Improved sequential-thinking tool documentation in `seqthink.rs`.
- Added safety filter documentation in `shell_filter` for credential and git operations.
- Enhanced utility tools documentation, including `cd`, `dir_cache_update`, and `todowrite`.
- Improved rendering documentation in view modules (`chat`, `markdown`, `status`, `workflow`) to clarify rendering flows and purposes.

src/model/agent_def/builtin.rs

@@ -1,5 +1,17 @@
+//! Hardcoded built-in subagent definitions (coder, reviewer, researcher, planner).
+
 use crate::app::subagent::spawn::AgentDefinition;
 
+/// Build the fixed list of built-in agent definitions shipped with zesdex.
+///
+/// Flow: construct each `AgentDefinition` with a name, system prompt, and
+/// allowed tool list, then collect into a `Vec`.
+///
+/// Why: these agents are always available regardless of global/session
+/// config, giving users a baseline set of roles out of the box.
+///
+/// Return: a freshly-built `Vec<AgentDefinition>` (coder, reviewer,
+/// researcher, planner).
 pub fn builtin_agents() -> Vec<AgentDefinition> {
     vec![
         AgentDefinition::new(