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.
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