feat: add background job runner, job__* handlers, and start gates

Detached tokio::process runner with a frozen JobEnvSnapshot (env-derived
bin dirs, vault-interpolated agent envs, COYOTE_TOOL_TIMEOUT resolved at
start), process_group(0) with pgid-guarded SIGTERM/SIGKILL escalation,
capture-only ring-buffer telemetry, and LLM_OUTPUT read after wait().
MCP jobs snapshot a single-entry McpRuntime holding only the validated
server and render through the same free fn as the foreground path.

job__start enforces its gates synchronously before any spawn:
jobs_enabled, the backgroundable whitelist with directionality teaching
errors, the per-request declared-names stash captured in
before_chat_completion, then capacity (lazy supervisor get-or-init in
plain sessions). job__check/list read the shared JobState cell without
consuming; job__collect blocks with the escalation early-out and applies
a tail-biased char-boundary cap plus optional tail_lines; job__cancel
kills the group with a 5s grace.

Job declarations are injected iff jobs are enabled at agent init, the
plain-session function-init sites, and the exit_agent rebuild; job__ is
carved out of enabled_tools filtering and excluded from
concrete_tool_names so REPL toggles cannot grant or revoke it.

src/config/input.rs

@@ -9,7 +9,12 @@ use crate::utils::{AbortSignal, base64_encode, is_loader_protocol, sha256};
 
 use anyhow::{Context, Result, bail};
 use indexmap::IndexSet;
-use std::{collections::HashMap, fs::File, io::Read, sync::Arc};
+use std::{
+    collections::{HashMap, HashSet},
+    fs::File,
+    io::Read,
+    sync::Arc,
+};
 use unicode_width::{UnicodeWidthChar, UnicodeWidthStr};
 
 const IMAGE_EXTS: [&str; 5] = ["png", "jpeg", "jpg", "webp", "gif"];
@@ -158,6 +163,14 @@ impl Input {
         self.data_urls.clone()
     }
 
+    /// Names of the function declarations this request will send to the model.
+    pub fn declared_function_names(&self) -> HashSet<String> {
+        self.functions
+            .as_ref()
+            .map(|functions| functions.iter().map(|f| f.name.clone()).collect())
+            .unwrap_or_default()
+    }
+
     pub fn tool_calls(&self) -> &Option<MessageContentToolCalls> {
         &self.tool_calls
     }