refactor: typed args for grep, bash, and vision tools

Convert remaining tools from manual args["key"].as_str() parsing to
serde Deserialize structs. Also removes the now-unused get_str()
helper from grep.rs and simplifies capture_tmux_pane() signature
(takes lines directly instead of re-parsing args).

All 7 tool modules now use the same typed args pattern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-agent/src/tools/bash.rs

@@ -7,6 +7,7 @@
 // display running commands and the user can kill them (Ctrl+K).
 
 use anyhow::{Context, Result};
+use serde::Deserialize;
 use serde_json::json;
 use std::process::Stdio;
 use std::sync::Arc;
@@ -16,6 +17,15 @@ use tokio::sync::Mutex;
 
 use crate::types::ToolDef;
 
+#[derive(Deserialize)]
+struct Args {
+    command: String,
+    #[serde(default = "default_timeout")]
+    timeout_secs: u64,
+}
+
+fn default_timeout() -> u64 { 120 }
+
 /// Info about a running child process, visible to the TUI.
 #[derive(Debug, Clone)]
 pub struct ProcessInfo {
@@ -94,8 +104,10 @@ pub fn definition() -> ToolDef {
 }
 
 pub async fn run_bash(args: &serde_json::Value, tracker: &ProcessTracker) -> Result<String> {
-    let command = args["command"].as_str().context("command is required")?;
-    let timeout_secs = args["timeout_secs"].as_u64().unwrap_or(120);
+    let a: Args = serde_json::from_value(args.clone())
+        .context("invalid bash arguments")?;
+    let command = &a.command;
+    let timeout_secs = a.timeout_secs;
 
     let mut child = tokio::process::Command::new("bash")
         .arg("-c")